Really frustrating situation but you're on the right track already. A couple of things that tend to help — make sure your document clearly shows your full name, school name, and a recent date all visible in a single clean shot without cropping. Highlighting is a good idea but sometimes the AI still struggles, so if you can get a document where all three details are naturally prominent that works better than highlighting on an existing one.
The enrollment letter is usually your best bet out of everything you tried — just make sure it's the most recent one issued by your university, ideally dated within the last few months. The syllabus and portal screenshots tend to get rejected more often since they don't always have your name and school name together on the same page.
Unfortunately there's no manual upload or email option at the moment, it all goes through the automated system. Waiting a couple of days and resubmitting with a cleaner document is genuinely the most reliable fix people have found here. If you keep hitting the same wall after a few attempts, drop back here with more details and someone may be able to help further. Good luck! 🙂

This is resolved. It appears that the issue wasn't the documents or the uploader. It was the recency of account creation, and that I was not viewed as an established legitimate user. After adding some profile details, followed by a couple of verified account links, I then became a confirmable Git user. The next attempt that I had made submitting an application, I was approved within 25-30 seconds. Now I have to allow for 72 hours to pass. Then the benefits should load into my account settings. Thanks all!