Q: my iphone 6s cannot send MMS why?

Hey Theint Nandar Tun,

I understand that you are having an issue sending SMS and MMS messages on your iPhone 6s. Let's see if we can get that sorted out. 

I would start by taking a look at the articles below. It will have you look over the settings on your iPhone to ensure that is correct. Other things to look out for is restarting your iPhone and checking for a carrier update. 

One thing to look out for is if this is for actual text messages or iMessages. You can tell the difference by the color of the message. If it is green, then the messages are going through your cellular provider. If they are blue, then they are routed through data and you will be communicating through Apple Devices. 

If you can‘t send or receive messages on your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT204065​

Use Messages with your iPhone, iPad, or iPod touch
https://support.apple.com/en-us/HT201287

Take it easy