Are you using the Samsung Messages App or another sms app ?
Are you sending normal texts or are you trying to send long texts or texts with pictures in as this would convert the text onto a mms which with some networks these cost to send and on occasion your mobile account may need provisioning and you need mobile data On.
Is this happening with all texts or a particular contact ?
This link may help.
I wish you all the best.
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 have no affiliation with Samsung.