Dear TM,
You need to click on Settings>List Settings to go to the List Settings page and then replace listedit.aspx with FormSettings.aspx like the following.
If you haven't tried it, I suggest you try it to see and let us know the result. Please check whether the Form Settings page URL is like the following in your environment.
If yes, what error messages pop up when you go to the page? Could you provide some related screenshots in your environment?
Note: Remove any private information before uploading the screenshots.
Welcome to share any updates when you have time if you need further help on this issue.
Sincerely,
Cliff | Microsoft Community Moderator