"He will be there at nine o' clock."
Translation:Θα είναι εκεί στις εννέα.
"εννιά" is corrected and accepted. If it wasn't accepted it means you had a mistake in the sentence.
You should have made a report.
TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM.
And check out the Greek Forum here with more links.
If you have any questions just ask.