QOBUZ app not working on Macbook

[Ein Heldenleben]

If About This Mac declares version 10.11.6 you have El Capitan.
If it shows 11.12.6 you have Sierra, and if 11.13.6 you have High Sierra.

You might be able to run High Sierra - but take care, as you'll clearly want to keep the apps you've bought with the machine.
I think most of the machines up to around 2012 can be updated to High Sierra. It's unlikely that a Macbook Air can be updated beyond that.

There are some programs which do definitely behave differently on Air models than on iMacs.

If you are happy with it as it is, and you are uncertain about installing a later OS version, then it's perhaps best to leave it alone.
OTOH, with a bit more courage and daring you could probably download High Sierra and install that to an external drive.
If that worked satisfactorily, then you could repeat the install to the hard drive on your Air.
Don't even think of doing that without taking some form of external backup.

Since you have other machines, I suspect that it's best for you to leave this one alone - specifically as it has a program you want to run on it which is currently working.

Thanks Dave . I suspect the previous owner downloaded High Sierra with a view to installing and then didn’t go ahead .