Unfortunately, not all tracks on Ultrafunk can be played for free, a few tracks require a YouTube subscription, either YouTube Premium or YouTube Music. These premium tracks are mixed in with all the free tracks and will be skipped over if you don’t have a subscription.
If you already have one of these YouTube subscriptions, the premium tracks should automatically work like any other free track on Ultrafunk. If not, you are probably not logged in to your YouTube account, please sign in first, then try to play the tracks again.
Disclaimer: Ultrafunk is not affiliated or associated with YouTube / Google or SoundCloud in any way.