Ιt depends on one's personal style (idiolect). Personally I find "περπατά" more "serious" and formal than "περπατάει". It's not absolute though, as there are regions where "περπατά" is used in both formal and colloquial types of speech. You will find such variations in a lot of verbs in Greek.
Look we moderators are here to help and we really try to do that. However, we cannot see what you wrote, not even what kind of exercise it was unless you make a Report.
So, when you have a problem you should first Report then comment here so we can see your Report immediately.
Here's how to report. Go to the bottom of the exercise page where you'll see REPORT Click on that:
Then choose what you want to REPORT: (You'll see these to choose from.)
-The audio does not sound correct.
-The dictionary hints on hover are wrong or missing.
-The Greek sentence is unnatural or has an error. -The "Correct solution" is unnatural or has an error.
-My answer should be accepted. This is the one you should click on if your translation was not accepted.
After choosing the REPORT you want to make you should post on the comment page.
It wasn't necessary to add "περπατάει" the system could see your answer and would show it to us, that's why it wasn't easy to find it had been sent to another area. No, problem I found it, but I still don't know what kind of exercise it was. As I said if it was a Listening exercise only one spelling would be accepted but if it wasn't a Listening they there might be a glitch with the program which we need to fix.