Translation:The plate is on the triangular wooden table.
- Although the "wooden triangular table" is odd-sounding to native English speakers, it is one of the accepted translations. Why your translation wasn't accepted is unknown since we are not able o see what you wrote.
- You should have made a REPORT, you must have had an error.
3.. what is not accepted is the language you use to refer to those of us who create these courses and moderate them.
https://forum.duolingo.com/comment/22424028 TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM
And check out the Greek Forum here with more links.
If you read the other comments on this page you will see that "the plate is on the wooden triangular table." is one of the accepted translations and if it wasn't accepted for you you should have made a REPORT.
There are instructions for how to make a Report in the same comment.