ID:251010 Can't create assignment -- can't write to Quartus Prime Settings File

CAUSE: You saved assignment edits in the Assignment Editor, but the Quartus Prime software cannot create the assignments. The Quartus Prime software cannot create assignments if one of the following situations exists:

The specified Quartus Prime Settings File (.qsf) is in use by another user.
The project and its files are designated as read-only.
The file does not exist.

ACTION: Verify that the Quartus Prime Settings File exists. You can save the file with a new file name or in a different directory. If the Quartus Prime Settings File is read-only, you cannot edit the project. Click the + icon to expand this message in the Messages window to see more details about the assignment.