I can’t send mms messages to non-apple users.

You need to contact your carrier. They are the ones that control this. You need to have an SMS/MMS plan on your account and have cellular data activated. SMS/MMS uses a portion of the cellular band.

MMS is controlled by your cellular carrier. You need to have an MMS plan from the carrier and you need to have cellular data turned on to be able to use it.

I can receive a message from a non Apple device but I can’t send one