ID:251009 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 Quartus Prime Settings File (.qsf) is in use by another user.
• The project and its files are designated as a read-only.
• The file does not exist.