I assumed that 'cover photo' meant the beige-like banner at the top of your profile page (where the 'edit profile' and 'cover photo' buttons are in the screenshot above)...
@Duma does clicking 'cover photo' also change your profile pic? Seems odd to have two buttons for that... Then again, much like @Granny63020, I don't actually have the 'cover photo' button at all, just the 'edit profile' one.
I never actually felt the need to edit my cover photo before, but now I'm invested in getting this missing button to appear.