JRay it seems to be the same problem that can be found here FriendsOfFlarum/upload246
The way we implemented it, if you installed the optional dependency for S3, then you must configure it. If you don't use S3 at all, you can use the command in the last post in the linked issue to remove it.
If you're using S3 as part of another extension, then you will unfortunately need to fill in the values in Upload even if you're not using S3 with Upload.