@LexiiBH Hello! I just replied to another post about this ...
EDIT: In the other post, the player mentioned they needed to find an appropriate MP3 converter. I wonder if your music is properly formatted. The files need to be in the MP3 format for sure.
I've never added custom music before so I just tested it. I downloaded a Pop song from Amazon and copied / pasted it into my Documents / Electronic Arts / The Sims 4 / Custom Music / Pop folder. Once I was back in the game, I went into the Game Options menu, selected Music, selected Pop and found my new song at the bottom of the list. I then unchecked all the in-game Pop songs so that only my new song was checked. I applied the settings and when I had my sim listen to the Pop station, the song played just fine.
Did you follow similar steps? If not, could you elaborate more on the steps you took to add the song?