Here are the most common and typical mistakes with the cove.tool Grasshopper definition:
Do you have any polysurfaces? They must be single surfaces only.
Do not edit the east vector. Edit the model position instead.
Can you make sure you have the latest version of IronPython as described in the troubleshooting article?
Can you make sure you are successful in logging into your account (make sure it is a company email and that your account is active),
Can you make sure you are uploading to the correct project?
Verify you have no errors in the script, then refresh the web app and select 3d.
After exporting to the geometry page, are you getting kicked back to the first project page? Read this article.
If you think you've done the above please recheck it again! :) One of the above things is not happening if you are not able to upload! See here for more a video tutorial: https://www.cove.tools/plugins