lock layers icon missing?

Hello, I don't usually lock layers so I'm probably asking a question with a very obvious solution... anyway, I'm trying to lock this layer and the lock icon is missing from the layers panel. When I looked up Adobe's instructions ( https://helpx.adobe.com/photoshop/using/moving-stacking-locking-layers.html ) for locking layers, the guide refers to the lock on the layers panel, and very clearly shows a lock icon, and this lock is absent from my layers panel. After a quick search in the Help menu, I found Layers -> Lock Layers, so I can do it that way, but I feel like it would be easier if I just had the lock icon back. Has anyone else experienced this?