Hey there, Cam_Br0!
It sounds like you're having issues with your iCloud Mail account, and this error message about it already being added to the iPhone.
It seems like the Mail feature is not enabled for the iCloud account, and that is why this error message appears. To check, go to Settings > Password & Accounts > iCloud.
Once here tap on "iCloud" and then make sure Mail is turned on or has a green button next to it. If it doesn't, turn it on then go to the Mail app and you should be able to access your iCloud mail account. It may take a bit of time for your mail data to sync to your device.
Here's more information if you need it.
Take care!