Cannot create a new primary iCloud email

I had created my Apple ID using gmail way back when, and now I wanna create an iCloud email as well. For whatever reason, I cannot see the "iCloud mail" option in my Apple ID settings or in my online iCloud account as well.

When I go to iCloud.com/mail, I get the following screen.

Why is it not available at all? Is this an iCloud+ membership thing? I highly doubt that because my family members use an iCloud mail account without any iCloud+ membership.

Any help would be appreciated. Thanks!