In the GitLab project page, when I click the wiki button it directs me to a page which creates a new Wiki page. I would rather see a list of all Wikis that has been created and an option below to create a new wiki.
Mine works fine. Chances are you don’t have the initial “home” page created and this is why.
For example: https://gitlab.mydomain.com/projectgroup/project/-/wikis/home
When that page exists, then it works fine.