To free up space you could delete any existing AP firmware that is not needed (i.e., any BSAP models you don't need to support and/or alternative firmware images), delete any packet captures (Status > Diagnostics > Traffic Capture), and purge all logs (Status > Logs).
If you still cannot upload the firmware after this, you will likely need you to open a regular Technical Support ticket by calling 888-423-8726 or sending an email to email@example.com so that we can troubleshoot directly on the unit. When you create a ticket, please note that this is in reference to an ADTRAN Support Community post.
I went ahead and flagged the "Correct Answer" on this post to make it more visible and help other members of the community find solutions more easily. If you don't feel like the answer I marked was correct, feel free to come back to this post to unmark it and select another in its place with the applicable buttons. If you have any additional information on this that others may benefit from, please come back to this post to provide an update. If you still need assistance, we would be more than happy to continue working with you on this - just let us know in a reply.