If this was your first partnership return OR you have never been late before ( you should have put the return on extension by 3/15) then you can write them a note and BEG for forgiveness of the penalty using the "I was stupid & didn't realize the deadline was not extended and I will never do it again" excuse (many will be using this excuse this year I bet) ... they may or may not waive the penalties but you can ask NICELY.
