Kenwood Car Radio PROTECT Mode

Hello everyone, I recently bought a used Kenwood car stereo, model KDC-4551U. So here's my problem: I connect it to my car (which is a 2003 Peugeot 206, by the way), it turns on fine, but it displays the word PROTECT and I can't do anything except insert and eject a CD. I've already tried resetting it with the button, unplugging and replugging it, and removing and reinserting the fuse.
It's worth noting that other car stereos work, and I should also mention that one of my speakers isn't working, but since it doesn't affect the other one, I don't see why that would be an issue.

So I think that's about it.
Thank you.