Translation:If you push back the meeting to Monday, tell us.
Because le is actually referring to the situation of postponement, not to the meeting itself.
Si... Indicates a conditional situation. The object pronoun (le) refers back to the entire clause beginning with si... This kind of reference to a conditional clause is invariably masculine.
"Rapporter" means to "take back", "bring back" or "bring in" which is not the same thing as to "put back".
"Remettre" is to "put back" and it can be used to mean "postpone", but I don't know if Duo would accept it as a reverse translation.
"Différer" is another such alternative that should be accepted as such but may not be.