Unable to delete sub folders in Mail on my iPad Pro

With which App are you having difficulty? Perhaps you are attempting to delete folders within your Mail App? Something else?

With more information as to what you are attempting to achieve, where, and how - contributors will be better able to provide appropriate guidance.

How, specifically, are you attempting to delete the folder previously created?

As an alternative to managing your mailbox folders from the iPad, consider signing-in to your Mail Providers webmail portal using a web browser.

Speaking of personal experience, your Mail Provider may restrict how mailbox folders are managed - and you may have considerably more flexibility in folder creation and deletion when managed from the webmail portal.