Hey there AJ_22xx,
You'll want to make sure you're signed in with the same Apple ID you're looking to use with iCloud Mail. You'll see this in Settings > [your name]. The email displayed at the top should be the email you want to use. If it's not, scroll to the bottom of that screen and tap "Sign Out". Once this is done, continue with the steps below and sign in with the correct account:
1. On your iPhone, iPad, or iPod touch, go to Settings > [your name] > iCloud.
2. If you want to send and receive iCloud Mail on your device, do one of the following:
• iOS 16, iPadOS 16, or later: Tap Mail, then turn on Use on this [device].
• iOS 15, iPadOS 15, or earlier: Turn on iCloud Mail (or Mail).
Set up iCloud Mail on all your devices - Apple Support
Take care!