Songs keep skipping like a broken CD when phone is locked

[Potatamort]

Whenever I lock my phone the music starts like skipping or stuttering and I've looked up about fixing this and it says turn off "WiFi optimization" in the settings but this literally isn't in my settings. I have a galaxy s6 edge if that helps but can anyone please help me solve this? Considering cancelling my premium if this can't be fixed because there's no point having it if I can't listen to my music properly.