Hi and welcome, Chickadee,
On a typical on-demand scan you'll probably get quite a few files, usually system files, which can't be scanned because "in use by another process". While you didn't say so, I presume that's what you're probably seeing. Totally normal, don't worry about it. If it really bothers you, try scanning in safe mode -- normally there should be fewer processes running that way, so avast can access more of the files.
And of course if avast caught something for you and you were able to get rid of it, so much the better.
Best,
Mike