"We don't take credit cards. We only take cash."
The point of the second sentence does not redundantly use 我們 as the whole part refers to 我們 without repeatedly using the subject.
So when it comes to sentence with commas, first take a look at the subject. Then, combine it together with the other parts, like this:
can be read as
I have reported-my answer had two sentences, as the original did. Was marked wrong, the expected answer had a comma, just one sentence. Just an error produced by being marked by a computer with only one answer option. I'm sure they will add other options in due time if people report it