A friend of mine was telling me ' I hope we can meet soon' , I replied in my email to her 'May be we could next month when you come to Prague'
The sentence sounds incorrect I don't know why. Is it wrong to use could when talking about the past?
Your sentence is just fine (except it should be "maybe" not "may be" ) 'Could' is often used to make suggestions: "we could go to a movie tonight" or "we could stay home". There is an element of condition there too, "we could go to a movie (if you want to)". Did you mean "talking about the past" or future?
Hope that helps