Browser iCloud login persistently prevented
My original xxx@mac.com does not allow iCloud login via my browser, and I want to be able to do that. When it asks me to verify my email address (xxx@mac.com) it displays an alert that says 'This email address is not available. Choose a different address.' I can't get past this step, even if I use xxx@icloud.com or xxx@me.com.
Please advise.
MacBook Pro 13″, macOS 15.7