How do I create a folder under email on my iPad?

In the sidebar "Mailboxes" section (you may need to tap on the sidebar icon at the top left corner to reveal the Mailboxes section), tap on "Edit", then on "New Mailbox" at the very bottom.

Give the Mailbox a name and select the account you wish to add it to.

click here ➜ Organize email in mailboxes on iPad - Apple Support

In Mail, display the list of all mailboxes, tap Edit, tap New Mailbox, set a name for the folder and set its location (a folder is owned by one of your email accounts.)