Hey there @PST1,
thanks for posting in the community !
You can read all about the "Local Files" feature on this help page.
However, i don't really understand what you mean by "adding my youtube music to Spotify".
Make sure you read all limitations and restrictaions provided on the help guide above.
I'm always around, let me know if you need further help 😃
Buddy, to fix that you need to click the button ". . ." in the left corner of Spotify, then you click "Edit> Preferences..." and go to the part that says "Local Files" and turn it on.
It worked for me so I hope it worked for you! ❤️