Greetings rhonwynfromeva,
We understand that you’re not seeing the options for the Mail app in Settings. This isn’t an option you can remove from the Settings app unless you have deleted the Mail app and not reinstalled it, restricted access in Screen Time or have a profile that restricts that access. Let’s check all of that.
First, do you have the Mail app installed on your iPhone? You’ll find it on your Home screen, in the App Library or by swiping down from the center of the Home screen and using Search to locate it. If it’s not on your iPhone, please reinstall it from the App Store.
If it is installed, let’s next check the restrictions in Settings > Screen Time > Content & Privacy Restrictions > Allowed Apps and be sure Mail is enabled.
Lastly, if neither of those change anything let’s check in Settings > General for an entry for Profiles or Profile & Device Management.
Let us know what you find out.