Do you have mobile data turned on? MMS will not work using only WiFi.
Did you get your device from your carrier? Is it branded to your carrier?
If mobile data is on, and you are using an unlocked device that isn't branded to your carrier, please tell us what carrier you have. The MMS APN might need to be edited.
Sent from my rooted Nexus 7 (2013) using Tapatalk