Thanks for posting and searching for the answer you need here!
If you're experiencing difficulties importing local files, as a first step please make sure that you're trying to do this using the steps here. Keep in mind that only files from official sources are supported. Once your tunes have been imported, you can download the playlist containing them as described in this Support Article.
If that doesn't do the trick, there're a couple of troubleshooting steps you can try out:
I have been having the same problem with local files being greyed out after working for a little while on my android for yearsm getting worse. Please don't tell people to do a clean install.. I don't think that has worked for anyone.
Moving the local files to another folder and going to the process of getting them on the phone again only works for a little while and it happens again.
Just admit it's an app problem and fix it spotify. Epic fail.