I received the message "You have an entry on Form 8283, Donee Acknowledgment which requires a donee's signature, electronic attachment is not supported."

[tswpow]

I have no knowledge of said form, or why I would need a signature for donations.

[Opus 17 · Accepted solution]

If you donated one item, or a group of similar items, with a claimed value of more than $5000, you must have a appraisal from a qualified appraiser, the appraiser signs form 8283, and you must also get form 8283 signed by an responsible person from the charity confirming the donation.  If you did not get an appraisal before a large donation, you can't claim a value of more than $4999.