1. Restart your device 
   • iPhone
   • iPad
   • 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 iMessage you need to ensure you can send message overseas. To send an SMS message, you need 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

For more information please visit Apple website.