When I send a picture it tells me to turn on mobile data. But its on already SM-S756-C Samsung. Can I get some help?

[bailenforcer]

when I send a picture it tells me to turn on mobile data.. But its on already SM-S756-C Samsung

I have a straighttalk SM-S765C phone, on Verizon/straighttalk network. Never saw this till this week. When I try to send a picture it says "please turn on Mobile data to send MMS. But it appears to be on. I have working wifi if that matters.

Can anyone help me figure this out.