Is there anything wrong with the following phrase?
a flea market at downtown of Singapore duirng a Street Festival

Question

Answer

Perhaps, "A flea market in downtown SIngapore during a street festival", note that "street festival" is not a proper noun so it does not need capitals.  Let us imagine it had been named "Charles" then it would require a capital letter.

Answer

It should be "A flea market in downtown Singapore during a Street Festival." but nice try!