Are you trying to open it in an incorrect version of Archicad. you normally see this error where you try to open a higher version file format in a lower Archicad version. Also, try launching Archicad app first and then navigate to the File, Open menu, locate the file and check the option in the open file dialogue to 'Open and Repair'.
I keep trying to share a link to an article on the Community site here but it keeps being removed. If you search for ‘cannot open plan file’ an article will show up that was published by Graphisoft explaining methods to fix the issue.
It seems the system was automatically quarantining the post because of the link - I don't know why, and it shouldn't have.
I have released one of them so the link is now there.