The 6d certificate is intended to protect potential buyers and their lenders from superior liens in the event of a legal action by the condominium trust. Without this certificate in hand you will have great difficulty convincing any even minimally knowledgeable buyer to proceed with the sale. Your condominium documents call for a unanimous vote of the trustees, this would ordinarily supercede the MGL (and the trust should so state). You do not mention the debt that the trustees are referring to. If it is an invalid debt or fictitious or they are acting out of malice you might threaten or pursue equitable remedies by going to court and compelling them to declare the reason for their withholding of consent. I sense that there is more to the is story and you should inquire deeper as to the trustees that are not consenting to better understand their motivation.