There is a bug where you miss out on being prompted for the initial barn find, and when this occurs you’ll never get notified for barn finds and never be able to acquire a barn find. I’m over level 50, done all 5 showcases, it’s a problem that will need a fix.
Since I never was prompted for a barn find I can only assume you get the first prompt around the time of the intro hour or two with the changing seasons. I was driving around a lot, getting influence from simply doing stunt jumps and skill chains. Next thing I know I’m qualified for my wristband, progressed in the game 10-12 hours and never received a barn find notification. I’ve stumbled upon what I guess are the barn finds while exploring but unless triggered you can’t get inside.