Hey @Els2002 ,
I have found this guide with some steps to try to fix this. Can you please have a look at it and see if that helps? You can access this from here: https://thedroidguy.com/samsung-mms-wont-send-android-10-1126541
Ease check your Apn settings to ensure the multimedia part is correct. Your Network's Customer Support Page's will show you the correct one's for your contract type.
In your phone look at…
Launch Messages , Press the 3 little dots in the top right hand corner, More Settings, Multimedia Messages, Attaching Unsupported Files and toggle On Allow.
MMS needs mobile data turned On.
Also sometimes your network needs to provision Multimedia messages to work on your account.
MMS are also typically charged to send and have a file size restriction.
Personally I use WhatsApp, Facebook Messenger or email to send photo's.
Daily Driver > Samsung Galaxy s²² Ultra 256Gb in Phantom Black.
The advice I offer is my own and does not represent Samsung’s position.
I'm here to help.