Sometimes you won’t see the “Claim this Knowledge Panel” button at the bottom. In that case it’s either already been claimed by someone or it’s not a claimable Knowledge Panel. Knowledge Panels that describe things that nobody represents or owns such as “lemons” are not claimable. Also, some that *should* be claimable aren’t, for example podcast Knowledge Panels.
Here is an example of a Knowledge Panel that cannot be claimed.
On the other hand, here is an example of a claimable Knowledge Panel.