I am sorry to ask, but I simply don’t get it.
There’s supposed to be a ‘add cover art’ button.
As mentioned on the page: https://musicbrainz.org/doc/How_to_Add_Cover_Art
But I am completely unable to see any such button.
There’s one named ‘set cover art’
That one yield the following:
“## Set cover art No releases have cover art marked as “Front”, cannot set cover art.”
And a set of tabs named ‘Overview, Aliases, Tags, Details, Edit’.
But no tab named ‘Cover art’.