Can you post a screenshot of the error (blur out any personal details or phone numbers)?
Does it only happen with a single specific number?
Were you ever able to text to that number?
Any chance you just joined 48 and activated your SIM?
If so then turn the phone off, take the SIM out, gently clean the contacts, reinsert it, switch on again and then manually search for networks and connect to 48 and try sending the text again.
Are you 100% certain that the destination number is correct, correctly formatted (e.g. +353 87 1234567) and correctly stored in your contacts?
If the problem persists then you may need to contact 48 support about this:
Hope this helps.