ID:251011 Can't remove assignment -- can't find assignment

CAUSE: You edited assignments in the Assignment Editor and then attempted to delete an assignment, but the Quartus Prime software cannot remove the assignment because it cannot find the assignment in the Quartus Prime Settings File (.qsf).

ACTION: Verify that the Quartus Prime Settings File specified in a submessage of this message and the assignment exist. If the Quartus Prime Settings File is read-only, you cannot edit the project. Click the + icon to expand this messages in the Messages window to see more details about the assignment.