Q: Can't send a message to a group.

Hello Heraveee,

Thank you for using Apple Support Communities. 

I see that you are having difficulties in sending group messages from your iPhone. I have a couple of articles for you to take a look at to sort out what might be going on. It sounds like you are not unfamiliar with group messaging, but I would suggest starting with the first on sending group messages, to ensure that you are not omitting something necessary. 

Send a group message with your iPhone, iPad, or iPod touch

After that, take a look at the steps in the following to eliminate other issues on sending/receiving messages. 

If you can‘t send or receive messages on your iPhone, iPad, or iPod touch

Best regards.