Maybe you got the package we were having a problem with lately.
To solve this first remove it with:
sudo apt-get purge -y apm-copter-erlebrain
Then update the keys for our packages:
curl packages.erlerobotics.com/keys/deploy | sudo bash
And install our latest stable package:
sudo apt-get install -y apm-copter-3.5-stable-erlebrain
After this you'll need to parametrize the autopilot, you can find instructions for that in the second part of this comment (the first part is for ESC calibration, which should already be done for the hardware we sent you).
If you still can't arm after updating the parameters please do try to calibrate the ESCs following the first part of the comment.