Hi ANNIELIZR,
Thanks for contacting Apple Support Communities!
We understand that you're unable to see your custom mailboxes in Mail on your iPhone.
Do you have more than one email account? If so, does this happen with every account that you add mailboxes to or is it just a specific account? If you need help to set up mailboxes, this article will guide you: Use mailboxes to organize email on your iPhone or iPad - Apple Support
"Create custom mailboxes
- In the Mailboxes list, tap Edit in the upper-right corner, then tap New Mailbox.
- Give your mailbox a name. If you have more than one email account set up on your device, tap Mailbox Location and choose the account where you want to create a mailbox.
- Tap Save, then tap Done.
You can find your new mailbox in the Mailboxes list under the account where you created it."
We look forward to your reply.
Cheers!