422
checkout.invalid_payment_linkcheckoutRule C9The payment link is not valid
One rule validates a link: a fixed amount must be positive, an open amount's minimum must not exceed its maximum, max_uses must be at least 1 and not below the uses already made, expires_at must be in the future, collect must be a known mode, and the amount cannot be changed once the link has been used. errors[].field names which.
Why the API refuses
WHEN a payment link is created or edited, THE SYSTEM SHALL reject a fixed amount that is not positive, an open amount whose minimum exceeds its maximum, a max_uses below 1 or below the uses already made, an expires_at in the past, an unknown collect mode, and an amount changed after the first use, naming the offending field.
What to do
Read the named field. The last condition is the one that surprises people: a link that has taken a payment is a historical record as much as a live object, so repricing it is refused — create a new link instead.