It's been a long time since I've seen this error and thought it had something to do with Windows Credential Manager but not sure.
-MS Project Professional M365 Version (Project Plan 3 license)
-Project Online - PWA
-User has licenses and can edit project in Project Center in browser - so not a license issue
-Project Pro license assigned to a user at domain X but the PWA license is a user at domain Y- so the app license is different from the license being used to connect to PWA.
When trying to add the account in Project Pro under "Manage Accounts", we get the following error:
Error text (for search): "Sorry, we couldn't find your license to connect to https://xxx.sharepoint.com/sites/pwa. If you know who manages the licenses for your organization, tell them you'd like a license to use https://xxx.sharepoint.com/sites/pwa".
NOTE: This works fine for all other users with this same scenario. User once had access but it has stopped working and we never get prompted for login credentials anymore.
Gotta love those errors that tell you nothing about the action problem that the software is having.