Opening old files InDesign

Leave aside the book document for a while i.e. the indb file and check the indd files which are supposed to be a part of it. I suppose they might have got corrupted. Indd files created in 2015 will open in 2020 version of InDesign. So try opening them and tell us if you get any errors or so. You may also check the filesize, it might be too small or too big if the file is indeed corrupted.

-Manan

Does your file still have the .indd extension on the end? If not, add it. Sometimes files lose their association to the app without the extension.

> thinking it would upgade automatically

No, InDesign doesn't upgrade automatically.

> Do they allow two different Versions.

Yes, you can have many versions of InDesign side by side.

Please clarify: what means "cannot open"?

Is it an INDB file? Are the INDD files still present? If you open the book or any of the documents in Indesign 2020, what happens?

"I have old 2015 on another macbookpro, but thinking it would upgade automatically" - Not, if you disable internet and then turn off automatic updates.