Hi John,
It is not clear what you want to open. You should either have a PLN, PLA, LBK, MOD or PMK. IIRC there are no other native ARCHICAD files. If the OS cannot guess how to open then you could try choosing the app to use by right clicking on the file and selecting “Open With” or you could open a new AC project and try to import the file in which is the procedure for PLTs.
Check here for a file extension list.
https://helpcenter.graphisoft.com/knowledgebase/25765/
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FYI when GS consolidated Plotmaker within AC (10? ) the option was to import and merge the PLN and the LBK going from to files to one. If at that time you only imported the PLN and didn't also merge the LBK without keeping the originals I think that you threw away the data you need now.
Eduardo Rolón AIA NCARB
AC27 US/INT -> AC08Macbook Pro M1 Max 64GB ram, OS X 10.XX latest
another Moderator