Usually issues about not being able to send MMS messages are because of incorrectly set APN.
> Tethering and networks
> Mobile networks
> Access point names
and select your appropriate carrier, if not, may need to be put in manually...
Only reason I can think of even though @DanielMena9
has said he's messed with those settings...
Name - Straight Talk
Apn - tfdata
MMSC - http://mms-tf.net
Mms proxy - 18.104.22.168
MMS proxy: mms3.tracfone.com
MMS port: 80
APN type: default,mms,supl,hipri,fota
Select the APN and reboot...