this is because the category page is missing on your wiki,
we have forget to include it in the english install process, Clement just create it on your wiki ( https://cohabitat-wikifab-prod.apps.cohabitat.net/index.php?title=Category:Tutorials&action=edit )
but it seems to have no change visible, you may have to refresh cache ( to do it, just edit LocalSettings.php file by adding empty line)
this may also be cause by the latest version of Flow extension, working only on latest mediawiki version. We are working on the update to mediawiki 1.28, you could wait that we done it to update on your side.