I cant create a icloud mail

See the article Create a primary email address for iCloud Mail - Apple Support

On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud.

Tap iCloud Mail (or Mail), then follow the onscreen instructions to create an iCloud Mail address.

If you don’t see onscreen instructions after turning on iCloud Mail on your iPhone, iPad, or iPod touch, you already have a primary iCloud email address.

leotopik Said:

"I don’t have iCloud mail I only have gmail"

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So, what does the error read?

Provide a Screenshot:

Providing a screenshot would most certainly help in this instance. IMPORTANT: Block out personal information (i.e. names) buy covering it with red-colored rectangles and such. Go Here: Take a Screenshot on your iPhone - Apple Support

leotopik Said:

"I cant create a icloud mail.: it’s giving me an error occurred during the process"

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So, what does the error read? You can only have one iCloud eMail account. After that, your only other options is to create an alias.