It shouldn't be a problem.
You should be able to open any version file back to version 10.
You might need to load migration libraries for it to be usable - but that is another issue.
Are you the only person working on this file?
If somebody else has it open and you are not team-working then the file will be locked.
You will see a 'filename.lck' file in the same location as the job file.
Sometimes these get left behind - so if you are the only person working on the file you should be able to delete it.
But even if this file does exist Archicad should give you the option of 'Open as read only' or 'Open with exclusive access'.
Can you show a screen shot of the message you get when you try to open the file?
PS I am assuming you are not running 19 in Demo mode.
One of the forum moderators. Versions 6.5 to 25 Dell XPS- i7-6700 @ 3.4Ghz, 16GB ram, GeForce GTX 960 (2GB), Windows 10 Dell Precision 3510 - i7 6820HQ @ 2.70GHz, 16GB RAM, AMD FirePro W5130M, Windows 10
Yes, I'm the only user, and no, of course I'm not using a Demo. I'll try again to post my screen-shot attachment that shows I don't have the 'Open with exclusive access' option that my previous ArchiCAD versions used to have. Thank you for your reply.