This article lists and describes common errors that may occur with the Project Online integration.
The service account configured in OneConnect to connect to Project Online does not have a license for Project Online. Please make sure you assign at least a Plan 3 type of license to the account.
The plan (named in the error) is currently checked out by the user in OnePlan. That person or someone else must check the plan back in for the item to sync data back to OnePlan or be updated with the values within OnePlan.