GitLab for Jira App not connecting (422 Error)

I am having a difficult time connecting the Gitlab for Jira app in Jira Cloud to our self-managed (free) instance.

I’ve configured all of the settings on our instance, but when I add our URL to the Getting Started button, I get the following message
“Failed to update the Gitlab instance” and in the browser console it shows me

I’ve looked at and referred to this post where some users had success but unfortunately the steps did not work for me: Gitlab Self Hosted to Jira Software

Any suggestions would be most helpful.