Is the problem creating an iCloud email address, or is the problem using it as the email address for your Apple ID?
If the latter:
Change your Apple ID - Apple Support
- There's special instructions if you want to change your Apple ID to an @icloud.com address.
"If you try to change your Apple ID to an @icloud.com email address that was created within the last 30 days, you will be asked to try again later."
If the former, you cannot change an @icloud.com primary email address. You can use an alias instead, but with limitations.
Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support - "An email alias is an email address that conceals your actual email address from a recipient. An alias can help you manage the email you receive, and monitor the sources of unwanted messages. You can both send and receive email from an email alias using Mail on iCloud.com.
"With iCloud Mail, you can have up to three active email addresses, and you can turn an email alias on or off at any time. You can also delete email aliases that you no longer want to use. While turning off an email alias is temporary, deleting it is permanent. "
With iCloud Mail, you can have up to three active email addresses,"
"Note: You can’t delete or turn off a primary iCloud Mail address. You can’t use an alias to create a separate Apple ID, and an email alias can’t be converted to a primary iCloud Mail address."
