Icloud mail account won't set up on I cloud mail

Re: "Why can't I set up a password and username for icloud mail"

Apple support have an article online that may help guide you:

Before you can use Mail on iCloud.com, you need to set up an iCloud Mail address on your Apple device.

How to: Create a primary email address for iCloud Mail.

NB: says: If you already have an @icloud.com (or an @me.com or @mac.com) email address,

see Set up iCloud Mail on all your devices.

Also:

How to: Use iCloud Mail on iCloud.com - Apple Support