There are some special considerations when GAMS has been installed on a network drive. Please run the GAMSIDE and select Help Topics from the Help menu. Then click on Installation Notes and read those.
This error also occurs, if your project directory is on a network drive that is not mapped. Try to map it or create a new project on a local disk drive.
Starting with distribution 22.4 the GAMSIDE will create a drive mapping when the project is on a network path.