Here's what Ballot Access News had to say:
... even though both the Green Party, and the Working Families Party, nominated him in May at their state conventions. The problem is that Platt also ran in the June Democratic primary, and narrowly lost that primary. State law seems to say that if a candidate seeks the nomination of several parties, and fails to get all of them, then he is entirely barred from the general election ballot. Platt will probably filed [sic] a lawsuit soon.
The decision was made Friday (June 27). We've previously covered the nuanced issue.
Update 1: In an e-mail Saturday night, Platt confirmed that a lawsuit is likely and said that he is in contact with "one of the country's best known organizations that seek to uphold the Constitution and the civil rights of everyone" about taking on the case.
(If we had to guess, we'd say it was the ACLU, though Platt wasn't providing a name.)
Platt also restated his case that state law only bars someone from running in the general election as a write-in or petition candidate, not as the candidate for another party that chose him as its nominee prior to his loss in the Democratic Party primary.