I was trying to uplod the following from furaffinity.net:

https://www.furaffinity.net/full/19348989/

However, it said the website was not permitted. HOWEVER, FurAffinity is, in fact, on the whitelist. Why is it not letting me post it if it's on the whitelist?

I just tried it with Hentai Foundry as well, with identical results.

The url you listed isn't an image, is probably why. Paste the "Full" link into your sources, but as direct url, use the download link.