I don't want to use iMessage on an iOS 11 phone. How do I turn it off?

Solution 1:

Turn off iMessage by going to Settings>Messages and switching off iMessage.

Then make sure "MMS Messagging" is turned on.

For more information check here:

Deregister iMessage on your iPhone

If you still have your iPhone, you can deregister iMessage with these steps:

1. If you transferred your SIM card from your iPhone to a non-Apple phone, put it back in your iPhone.

2. Make sure that you're connected to your cellular data network.

3. Tap Settings > Messages and turn off iMessage.

Solution 2:

My guess is that while iMessage is turned off on your phone, you are trying to send SMS to iMessages addresses (which ca be either mail addresses or phone numbers). If you previously sent iMessages to contacts using mail address for their iMessage account this could explain the error message you got.

Can you try the following?

• When you're about to send a message to a contact that triggers an error, click his name (at the top of the message window) and check which identifier is used (mail or phone number). If it is not a phone number then start a new conversation using it.

You can start a new conversation by clicking the "new Message" button at the top right corner. Then, type your friend's name and click the one with the phone number.