X_STATE_TRANSITION_ILLEGAL
Means
the machine has no such transition
Typical cause
three conditions and one code, because they share one repair — the move is not in the table — and cause states which. "<from>" is not a state the enumerated() set declares; the row is terminal in "<from>", an empty move list being how a terminal state is written; or the move to "<to>" was never declared. The unknown-state branch is separate because an unknown state has no outgoing moves either, so a single "no legal moves" answer reported a typo as "the row is terminal in <typo>" — a sentence about a state that does not exist. 422 and not 400: the request is well formed and its schema passed, which is X_INVARIANT_VIOLATED's shape and takes its status. Refused before any statement opens a connection, so nothing was written
Fix
the fix is the same call with a legal target substituted — orders.transition('status', id, { from: 'pending', to: 'paid' }) — or add "<to>" to the "<from>" entry of the transitions() table