"I know what you had done."
Translation:Ξέρω τι είχες κάνει.
Of course, the pronoun is not the error. But since we can't see the rest of the sentence we are unable to tell you what the mistake was. You should have made a REPORT.
TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM https://forum.duolingo.com/comment/22424028
Please do not post comments about rejected sentences if you have not first made a REPORT