There are a couple of possibilities. One is that the file you're trying to open isn't a Final Draft file. FD can open .fdx files and can import .rtf files, .txt file and (FD13 and 12 only) .pdf files. An attempt to open any other file type will return this error message.
Another possibility is that the file has become damaged. If this is the case, you can retrieve a backup copy that FD makes every time you save the master. See Where is the Final Draft backup folder? for information and short video on accessing the backup folder.
Check the file size:
Mac:; Right-click or Control + click the script file icon and choose Get Info;
Windows: Right-click the script file icon and choose Properties.
If the file has more than a few kilobytes or megabytes, it may be fixable. Open a ticket with a technician to see if the file can be repaired: https://fd.support/3dTsV8R
If the file reports 0 kilobytes or 0 bytes, it's empty and a backup should be used.
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https://fd.support/3oOHekJ