First things first, from the video it would appear that when you disconnect from your computer (which you must - the motors won't arm whilst it's connected to the assistant on your computer) and power cycle, your LED is flashing rapidly red continuously with no pause, correct? The manual is clear that this is one of two things, neither of which are good: (1) Calibration or other error or (2) Second level protection low battery warning. You say that the low battery warning is off so we can rule out number 2, though it would be wise to check.
So, I would suggest you perform compass calibration. I can't honestly say I know that without it it won't arm the motors when the compass isn't calibrated but it seems likely, given that the LED is clearly indicating an error condition. Follow the instructions in the user manual for calibrating the compass. You will need to mount the compass/GPS sensor before you do it.
Once you've done this, try again. If that hasn't cured it, you need to re-check the calibration of your transmitter. The transmitter is clearly working because you can see the responses in the assistant. First, set all the trims to completely neutral/zero/centred position. When you start calibration, don't worry about where the sticks are, what the assistant shows, or anything else. Just click the button to start and move the sticks several times through the full extent of their range. Don't worry that the sliders don't show the full range on the scales on the screen, just move the sticks full through range. Then click finish. Now centre all the sticks. Are all showing in the middle, in green? If not your transmitter is a bit out of alignment, in which case reply and we'll sort that out. Now, move each stick to its extreme and make sure that the indicator moves fully to the end of each scale. It has to go right to the end, otherwise the command to arm the motors won't be received. Verify it's also working in the correct sense, i.e. move throttle fully UP (full power) moves the slider fully to the right, moving throttle fully DOWN (zero power) moves the slider full to the left. Check the others are in the correct sense too - for pitch moving the stick up should move the slider right and for stick down the slider should move left. Once you've done these tests and everything is well, click Write on the menu bar to save the changes to the flight controller. Disconnect the USB cable from the flight controller, power cycle and try again. The slightest lack of extreme position, e.g. caused by subsequently moving the trim a few notches in the other direction, can prevent it arming, as can setting a power curve which doesn't allow the channel to go to the extreme, using dual rates that don't allow the channel to go to the extreme, etc. etc. If your Transmitter has those features check those.
You mention calibrating the ESCs for maximum throttle in the video. This has nothing to do with arming them or not. However, before you fly you DO need to do the calibration, but you can't do it with them connected to the flight controller. You need to unplug all of them from the flight controller, and calibrate them one at a time by plugging them directly in to the throttle channel of your receiver. Once they're all done (not forgetting to also set the battery type to NiMH, ***NOT*** LiPo even though you're using a LiPo battery) you can reconnect the ESCs to the flight controller, paying particular attention to the ordering of the motors; check the manual for the correct sequence.
So, I would suggest you perform compass calibration. I can't honestly say I know that without it it won't arm the motors when the compass isn't calibrated but it seems likely, given that the LED is clearly indicating an error condition. Follow the instructions in the user manual for calibrating the compass. You will need to mount the compass/GPS sensor before you do it.
Once you've done this, try again. If that hasn't cured it, you need to re-check the calibration of your transmitter. The transmitter is clearly working because you can see the responses in the assistant. First, set all the trims to completely neutral/zero/centred position. When you start calibration, don't worry about where the sticks are, what the assistant shows, or anything else. Just click the button to start and move the sticks several times through the full extent of their range. Don't worry that the sliders don't show the full range on the scales on the screen, just move the sticks full through range. Then click finish. Now centre all the sticks. Are all showing in the middle, in green? If not your transmitter is a bit out of alignment, in which case reply and we'll sort that out. Now, move each stick to its extreme and make sure that the indicator moves fully to the end of each scale. It has to go right to the end, otherwise the command to arm the motors won't be received. Verify it's also working in the correct sense, i.e. move throttle fully UP (full power) moves the slider fully to the right, moving throttle fully DOWN (zero power) moves the slider full to the left. Check the others are in the correct sense too - for pitch moving the stick up should move the slider right and for stick down the slider should move left. Once you've done these tests and everything is well, click Write on the menu bar to save the changes to the flight controller. Disconnect the USB cable from the flight controller, power cycle and try again. The slightest lack of extreme position, e.g. caused by subsequently moving the trim a few notches in the other direction, can prevent it arming, as can setting a power curve which doesn't allow the channel to go to the extreme, using dual rates that don't allow the channel to go to the extreme, etc. etc. If your Transmitter has those features check those.
You mention calibrating the ESCs for maximum throttle in the video. This has nothing to do with arming them or not. However, before you fly you DO need to do the calibration, but you can't do it with them connected to the flight controller. You need to unplug all of them from the flight controller, and calibrate them one at a time by plugging them directly in to the throttle channel of your receiver. Once they're all done (not forgetting to also set the battery type to NiMH, ***NOT*** LiPo even though you're using a LiPo battery) you can reconnect the ESCs to the flight controller, paying particular attention to the ordering of the motors; check the manual for the correct sequence.
Will do that tommorow