If you're on the initial steps of setup and trying to click the Register Me button or any other Save button on the Configure pages but they're not clickable or they're greyed out. You're probably just missing a permission set.
You need to have the "Appointiv Administrator" permission in order to click the Register Me button.
Go to this article to see the full step you missed.
Install Step 3: Permission sets - assign Users as Admins, Service Providers or both: https://support.appointiv.com/hc/en-us/articles/228230847?source=search&auth_token=eyJhbGciOiJIUzI1NiJ9.eyJhY2NvdW50X2lkIjoxMDc1MDY4LCJ1c2VyX2lkIjo0NTU1OTQ5MTIwNzgzLCJ0aWNrZXRfaWQiOjE5NTAsImNoYW5uZWxfaWQiOjYzLCJ0eXBlIjoiU0VBUkNIIiwiZXhwIjoxNjUxMTMxMjk2fQ.H6gOu1pZYGB_aZIul3YleHX1vGECo7Ekr0yQVg8KwIY
Please give yourself the Appointiv Admin permission and that should solve the issue.