Account being used somewhere else
Spotify is licensed for personal use, so you can only play from one device at a time.
If you see a message that your account is being used somewhere else, you just need to select the device you're currently using.
You can also tap Devices Available in the Now Playing bar (on mobile) or click (in the bottom-right on desktop) to select the device you want to play on.
I don’t recognize a device using my account
You may have left your account logged in on someone else’s device. If you don’t have access to that device, you need to ask the owner of the device to log out of your account.
Tip: If you’re concerned someone has taken over your account, get help here.