You may not have an SMS plan enabled by your Carrier. Contact you carruer and get an SMS/MMS plan enabled for the SIM that you use.
Types of group messages
The three types of group messages that you might see are: group iMessage, group MMS, and group SMS. The Messages app automatically chooses the type of group message to send based on you and your recipients' settings, network connection, and carrier plan. For example:
- If you and your recipients are using Apple devices and iMessage is turned on, then the group message is sent as an iMessage.
- If you send a group message to people who aren't using iMessage, then the message is sent as an MMS or SMS message.