In no particular order…
If you have an add-on VPN client app in use, discontinue and remove that.
If you’re using the developer website to enroll, try enrolling via the developer app.
If you’re in India and using the website, you’ll need to use the app.
If you have a mobile number format Apple ID, those usually haven’t worked and an email-format was (is?) needed.
If you’re in an embargoed or restricted region or country, that enrollment won’t work.
If you’re a denied person, that enrollment won’t work.
You’ll need a major payment card associated with your Apple ID.
If one of those is not the case here…
Contact Apple Developer Support.