Mms may need provisioning on your mobile phone account which your Network's Customer services need to do @garydcfc1088
Also mobile data needs to be on with a steady 3G connection.
Mms are usually chargeable too to send.
Look at your apn settings in your phone and make sure the mms part is correct. Check on the support pages for your network to see what they should be.
In your phone launch messages and press the little dots in the top right hand corner and choose Settings, more settings, multimedia messages, and under restrictions choose free.
Also to add as mms files are size limited I prefer to use WhatsApp, Facebook messenger or email.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.