Cannot create new mailbox.

Hello ashthers,

Thank you for reaching out to Apple Support Communities, and we'll be happy to help in any way we can. We understand you're having issues with creating or renaming an existing mailbox on your iPhone. Follow the steps listed here to create your mailbox: Use mailboxes to organize email on your iPhone, iPad, or iPod touch - Apple Support

"How to use mailboxes 

A mailbox is a folder that you can use to organize your email. In the Mail app, you can use standard mailboxes — like Inbox, Sent, and Trash. Or you can create custom mailboxes. You can also use VIP and flags to make certain email messages stand out.

Create custom mailboxes

1. In the Mailboxes list, tap Edit in the upper-right corner, then tap New Mailbox.
2. 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.
3. Tap Save, then tap Done.

You can find your new mailbox in the Mailboxes list under the account where you created it.

Move your email messages to a mailbox

1. Go to your Inbox in Mail.
2. Tap Edit in the upper-right corner.
3. Select the email messages, then tap Move.
4. Choose a mailbox to move the email messages to.

Delete mailboxes

When you delete a custom mailbox, it permanently deletes all of the email messages inside the mailbox and from your account. Before you delete a mailbox, you can move your email to another mailbox.

To delete a custom mailbox:

1. Go to your Mailboxes list and tap Edit in the upper-right corner.
2. Tap the mailbox you want to delete.
3. Tap Delete Mailbox.
4. Tap Delete, then tap Done.

You can only delete custom mailboxes.

How to use the VIP mailbox

In the Mail app, you can specify certain people as VIPs to help keep track of messages they send. Your VIPs have a star icon  next to the names in the message header. Your VIP mailbox is in the list of your mailboxes.

If you can't find your VIP mailbox, make sure it's turned on. Go to Mail, tap Edit, then make sure VIP is selected in the list of mailboxes.

Add a VIP 

1. Go to an email.
2. Tap the sender’s name or email address in the message header.
3. Tap Add to VIP.

Mail adds the VIP to your VIP mailbox.

Change VIP alerts

1. Open Mail.
2. Tap the Info button  next to VIP.
3. Tap VIP Alerts.
4. Then select the alerts you want to set.

Flag your email

You can mark your email with a flag to help you keep track of your messages. You can even color code your different flags. When you flag an email, you’ll see a flag icon  next to the email thread.

Use the default flag:

1. Go to the mailbox that has the email you want to flag.
2. Swipe left over the email.
3. Tap Flag.

To change the color of a flag:

1. Go to an email thread.
2. Tap the Reply  button.
3. Tap Flag, then select the color you want to use for that flag.

Find your flagged email

1. Open the Mail app.
2. Tap Edit in the upper-right corner.
3. Select Flagged.
4. Tap Done."

We hope this helps.

Cheers!