Hey!
From my experience, this usually occurs when an asset fails validation (as indicated by the error code: errors.com.epicgames.cookplugin.assetvalidationerror).
If you have imported any assets (images, meshes, etc.), try the “Validate Project” option located in the menu after clicking the 3 dots next to the “Launch Session” button.
If that still does not work, try the “Upload to Private Version” option. This will usually provide a more detailed breakdown of the assets that are failing validation in a message log. You should then be able to manually validate/fix up the conflicting assets.
Also, as a side note, make sure your project is under 400MB.
Let me know if this helps!