"I make a cake."
Translation:Εγώ κάνω ένα κέικ.
You seem to be implying that your sentence was rejected and you are assuming we hadn't added "φτιάχνω κέικ" but it was added. That now leads us to believe that you had a mistake in your sentence. So, now you need to see where your mistake was. Have you made a REPORT?
See here for how to make a Report etc.
TIPS TO MAKE LEARNING EASIER + HOW TO REPORT A PROBLEM https://forum.duolingo.com/comment/22424028
https://forum.duolingo.com/topic/936 If you have any questions just ask.
And please don't comment about rejected sentences if you haven't first made a REPORT.
On a recent one of these κάνω / φτιάχνει questions, Duo suggested ONLY φτιάχνει in the drop down box BEFORE I submitted my answer then gives κάνει as the answer ... AND the drop down box now suggests both verbs! ... AND I took BEFORE & AFTER screenshots to prove it because I've been tricked by it before and I thought you wouldn't believe me unless you saw it for yourselves.
This time I deliberately wrοte Κάνω ένα κέικ because the drop down box said that κεικ is neuter and was marked wrong.
Their needs to be some consistency rules set up and enforced by the developers to avoid unnecessary confusion and waste of YOUR time having to explain it repeatedly to us.