It's all about time (OR a time?)

Hello.

I have a question concerning a grammar point. I am an English teacher.

Recently, my students took an English test at my school. One of the questions was a short (2-3 sentences) essay about things they did recently.

One of my student's answers was this, "We had good time yesterday". I know this is wrong and it should be "We had a good time yesterday". But I don’t know how to explain to my student to teach them that it is wrong.

For example, “I have time to talk” or “I don’t have time now” are grammatically correct sentences without using “a”.

Why are the above sentences OK but “We had good time yesterday” is not good grammar?

Thank you everyone for your replies in advance!