I understand that you can't delete a mailbox in your Mail settings. It's important to make sure that you can manage your mailboxes, so I'm happy to provide some help.
I suggest using the 'Manage your mailboxes' section of this support article: Use mailboxes to organize emails on your iPhone, iPad, and iPod touch .
Additionally, I would like to share with you the Mail Support page that has all the topics, resources, and contact options you need for the Mail app on your device.
Thanks for reaching out to the Apple Support Communities.
Have an amazing day!