You may need to speak with your network provider customer services team to have mms provisioned on your account.
They are a chargeable service for each mms sent and a good 3G data connection is needed too.
Also they are limited to a file size restriction.
I prefer to use email or Facebook Messenger or WhatsApp as it's free to send this way.
Current Phone ~
Samsung Note 20 Ultra 5G SM-N9860_256Gb SnapDragon 865+
Samsung One Ui 3.1/ Android 11.