The Bug
The /playsound doesn't register with Minecraft. When you use it, despite that its accessible using tab and has arguments, Minecraft errors out and says "unknown command".
Also affects versions...
This bug also affects every version between 1.13 and 1.15 pre-6, but this website would only let me select the versions shown above. :|
Please provide the exact command used.
Also make sure that you have cheats enabled in your world.