Λιγότερο is "less" in English. The sentence above means "the team has one less member". There is also this way to say that it english "The team is short one member". Short in the meaning of not tall is κοντός in Greek. The team has one short member would be Η ομάδα έχει ένα κοντό μέλος, which has a really different member.
"The team has one member fewer."
is one of the accepted translations. In fact, it's number six on the list of 117 accepted translations.
Without seeing the whole sentence we can't begin to imagine what you had wrong. Did you make a report? I can't see any.
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:
-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.