It won’t let me add certain adobe fonts and media and stock images from other Adobe products

It should work, unless you’re on a free tier. 
If you’re on a paid tier, you should have access to this
 

See the little crown next to the “upload” button? It means I’m on a premium subscription and therefore I can upload fonts I obtained the licence for (This last thing is very important, whether free or paid for, you need to have the licence for the fonts you are uploading).

Regarding stock Photography, it’s a bit the same some images are available for the free tiers, but most require the premium subscription to be able to place them in the pages. However, you generally can only see what available for you in the assets section, so it shouldn’t be a problem. As ​@N8. suggested, a more detailed explanation of the steps you are taking as well as a few screenshots would be useful - as for the moment we don’t know how you are accessing the images, where exactly they are in your other “Adobe products” (which products?) and your exact workflow so we can’t really check why you are restricted in this way.

A clarification of the device you are using would also help!

​@Mia_Bookstaber7372  To be able to help you further,
 Could you create a full screen video of your workflow showing us what is happening? This would help considerably.
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Nate