I have a arbitration case with the MTA , I sign a stipulation and agreement , 1 yr and 6 months ago, can the stipulation be challenge

The grounds for challenging a stipulation would be the same as with any other contract - fraud, duress, lack or failure of consideration, lack of capacity, mutual mistake, etc.  You don't specify on what grounds you believe the stipulation is invalid, but you should know that these defenses are not often successful, and are less likely to be successful the longer you wait to challenge the contract.  You don't specify the circumstances under which you signed the stipulation, but if you were represented by a lawyer when negotiating and drafting the agreement, and/or the agreement was endorsed or approved by the arbitrator, your chances on prevailing in your challenge become even smaller.