Click on the "View all the required fields" link to see all required fields, and make sure all required fields are added to the registration form; otherwise, the registration form can't be saved.
You can put the registration form on any page; it will work, except that you are using the "Official GDPR compliance" module, or other modules that add required fields to the form and have restrictions to work on the registration page only.
Note that the "Official GDPR compliance" module works on the registration page only, so it can't be shown on that list. If you are going to use it, then add it to the registration form, because it adds a required checkbox to the registration form.
To make the Official GDPR compliance module work on every page, you need to customize the /modules/psgdpr/psgdpr.php file for a bit, check the 3rd image below.
