why my iPhone message doesn't give the option to name the group?

Hello LucyS14 and welcome to Apple Support Communities.

I see you have a question about naming group message threads in the Messages app.

Send a group message on your iPhone, iPad, or iPod touch - Apple Support

Tap the top of the thread where all of the participants are listed and you'll see options including Info. Tap Info. If all members of the group are using the iMessages protocol (i.e., they are all using Apple devices), then you'll see a field called Name where you can name the group. If even one member of the group if using an Android device, then the thread defaults to standard SMS text message format. Group SMS messages are not able to be named.

Cheers.