I have installed Wikibase on my custom Mediawiki, but when I look at an item, it does not show the "add statement" button under statements (In the picture this is the German "Aussagen"). However, I am pretty sure this button should be there in an out-of-the-box Wikibase install.
It did this in 1.39, so I updated to Mediawiki 1.41 and reinstalled all extensions for 1.41. The install according to the instructions seems to have gone without a hitch - Wikibase shows up as an installed extension, I can add items, and I can see all special Wikibase pages. The dependencies are up to date. Some of the maintenance scripts did funny things, see below. However, I do not think I need any interwiki things, since it is one Wiki and one Wikibase and that is it.
$ php maintenance/run.php ./extensions/Wikibase/lib/maintenance/populateSitesTable.php
Got no data from https://meta.wikimedia.org/w/api.php
done.
$ php maintenance/run.php ./extensions/Wikibase/repo/maintenance/rebuildItemsPerSite.php
Start rebuild...
Processed 23 entities.
Rebuild done.
$ php maintenance/run.php ./maintenance/populateInterwiki.php
Error during fetching data.
I also tried to update submodules, as this is also outlined in the Wikibase Readme, but I got these errors.
$ git submodule update --init
/usr/lib/git-core/git-submodule: line 7: basename: command not found
/usr/lib/git-core/git-submodule: line 7: sed: command not found
/usr/lib/git-core/git-sh-setup: line 86: basename: command not found
/usr/lib/git-core/git-sh-setup: line 86: sed: command not found
/usr/lib/git-core/git-sh-setup: line 332: uname: command not found
fatal: not a git repository (or any of the parent directories): .git
On Special:Version, this is listed: Software Version MediaWiki 1.41.0 PHP 8.2.15 (fpm-fcgi) ICU 63.1 MySQL 5.7.42 Lua 5.1.5