Hi there 
That error would result from your Game/User Data being over threshold, rather than the Assets Library quota being over threshold.
If you check here The Forge, do you have sufficient space available in your Game/User Data?
If not, please check here The Forge View Data Storage to clear space by deleting unused packages. The Others tab might contain large intermediary files like zip files which you would be able to clear.
Please see Managing storage on The Forge - Your Asset and Data Library for more info, and let me know if you still encounter an issue.