“Not Delivered” Message Still Appearing

iMessage doesn't say “Delivered” on your iPhone may due to the person you sending the message to has a non-iOS device. In this way, if you fail to send a message, you should resend the message as a text message by enabling Send as SMS in Settings on your device (Settings > Messages > Send as SMS).

If you can't send or receive messages on your iPhone, iPad, or iPod touch

Learn what to do if you see an alert when you send a message or if you can’t send or receive messages.

Try these steps first

1. Restart your iPhone, iPad, or iPod touch.
2. Check your network connection. To send a message as iMessage or MMS, you need a cellular data or Wi-Fi connection. To send an SMS message, you need to have a cellular network connection. If you turn on Wi-Fi Calling, you can send SMS messages over Wi-Fi.
3. Check with your carrier to see if the type of message you’re trying to send, like MMS or SMS, is supported.
4. If you’re trying to send group MMS messages on an iPhone, go to Settings > Messages and turn on MMS Messaging. If you don't see an option to turn on MMS Messaging or Group Messaging on your iPhone, then your carrier might not support this feature.
5. Make sure that your device has enough space to receive images and videos.
6. Make sure that you entered the correct phone number or email address for the contact.

If you see a Waiting for Activation alert, learn what to do.

If you still need help, find your issue below to see what to do next.

If you can't send or receive messages on your iPhone, iPad, or iPod ...

https://support.apple.com/en-in/HT204065