Be natural, this can be annoying if you're apologizing too often, write your message in plain language—like a human talking to another human. If error is caused by you (your app, your server e.t.c.) add apology, otherwise leave just statement of fact and how to resolve the issue.
For example:
- "Sorry, we couldn't send your message because of problem on our server. Please try again."
- "We couldn't send your message, please fill email field."
Another one:
- "Sorry, you don't have permission to access this feature anymore, your balance was low and we downgraded your plan."
- "You don't have access to this feature, it's available only in our Extended plan."