How to create a folder in mail app on iPad.

Presumably you are using an IMAP email account.

Individual mail folders are somewhat confusingly known as “mailboxes”. Each email account will have a number of default mailboxes (folders); in addition to default mailboxes, you can add additional mailboxes - however, IMAP does not allow you to create a mailbox (folder) within a mailbox. As such, while additional mailboxes can be added, the structure will be flat.

See >>> Use mailboxes to organize email on your iPhone, iPad, or iPod touch - Apple Support

In the Sidebar tap on Edit > New Mailbox and create a new mailbox name then mailbox location and save.