Hey guys/gals was hoping I could get a bit of clarification. We bought a condo recently (old house that was updated into 3 units) and the doorbell doesn't work on it.

I've checked the wires on the doorbell button itself to make sure it wasn't a button issue, the chime bells aren't stuck and seem to be easily moveable, so I'm thinking its a transformer issue

The only thing is I can't find a transformer, the only thing I'm finding by the circuit box is a brown wire with a label on it that says "doorbell transformer"...

This has me a bit confused, does this mean a transformer was never installed? shouldn't there be 2 wires?

I've attached a photo for clarification if that helps.
Thanks in advance!