What happens if a supplier tells me a contract was never actually assignable like the seller promised?

If the seller represented in the purchase agreement that this contract was assignable — a standard representation in most deals — and it turns out the underlying supplier contract actually required consent that wasn't obtained, or wasn't even obtainable, that gap generally supports an indemnity claim for breach of that representation, subject to the negotiated survival period, cap, and basket that apply. How significant the supplier relationship actually is to the business also affects how the claim is valued, since losing a minor supplier is a very different problem than losing one the business genuinely depends on.

If this issue is caught before closing instead, it's typically treated as an unsatisfied closing condition — assignment or consent not obtained — giving you the option to delay or walk away rather than closing and relying on a claim afterward. Since you're discovering this only now, confirm exactly what the purchase agreement said about this specific contract's assignability, since that representation is the actual foundation for whatever claim you have.