I had the same problem with a pack of powerpoint slides I had worked on over time. In the end, I had to change all fonts to free ones. Even then, I still couldn't download. I had to download each page one after the other to see which pages were still affected. A very long and painful process. I think the only solution would be for Adobe to prevent content creators from using a "bait and switch" tactic of making content free for a while then switching to paid. Even if they are allowed to switch to paid mode, this shouldn't retrospectively apply to those who have already used the content. It's really not fair how it is being done currently.
For those with Education accounts running the "paid content" error on download, could you please try again now (if you have a browser window open on Adobe Express, be sure to refresh it first)?
I assume you are using an Education account? If so, we are aware of this issue, and are actively pursuing changes to unblock users facing this "paid content" issue.
If you need to be unblocked immediately, howerver, you'd need to figure out which fonts in your file are premium, and will need to change them to free ones. Feel free to DM me if you need help in this regard.
I am having the same problem. Unable to download because project contains "paid content". I created every detail on the project, and do not see the paid content symbol anywhere. Screenshot uploaded below.
There are many free assets on Adobe Express, but there is also paid content. You can tell if it has a little crown icon in the right-hand corner. If you have a Creative Cloud subscription you might also see a check mark on the crown, indicating access to the paid content. Aer you using the free version of Adobe Express? Do you have a subscription?