Welcome to our Community! We'll need a bit more info to get to the bottom of this for you.
Have you ever been able to send messages to that number in the past? Are you currently able to call that number?
Can you also tell us what phones you're both using?
Is it possible they may have blocked SMS from your number at a certain point and forgot that it had been done? You may want to ask the recipient to check.
Keep us posted.