Code	Description
1011	Missing `AMT`
1012	`AMT` too low
1013	`AMT` too high
1031	Invalid `REF` — wrong length or an unsupported character
1040	Invalid `FOLLOWUP`. The value was longer than 4 characters or contained a non-letter character.
8300	General request format error: The TEXT will contain a description of the error
8301	Duplicate `SCHEDID`
8302	Missing/invalid Invoice
8304	Missing/invalid PYMT data
8305	Missing/invalid card data
8306	Missing or invalid `SCHEDID`. On `StartSession`, also returned when the `SCHEDID` is under 8 characters or does not exist.
8307	`SCHEDID` not found (schedule operations)
8308	PYMT (with specific `REFNUM`) not found
8309	PYMT locked: this payment cannot be processed, updated or removed because of its status
8310	PYMT not updated/deleted: This payment was either deleted or needs to be updated
8311	Schedule locked. Which statuses are locked depends on the operation - see below.
8312	Schedule not ready: Cannot process payment unless schedule has card attached
8313	Invalid `SUBTYPE` for payment processing
8314	Invalid `AMT`
8399	Schedule error: Internal gateway error while saving the schedule, check the status of the schedule to confirm the actual state after the incomplete save
8421	`PROGRAM` format invalid — missing or malformed `NAME`
8422	PROGRAM/PLAYER/ACCOUNT `AMT` invalid — must be greater than zero
8423	`PLAYER` format invalid — missing or malformed `ID`
8424	`ACCOUNT` format invalid, or a PROGRAM/PLAYER/ACCOUNT breakdown totals more than the payment `AMT`
9013	Invalid `REFNUM`
9100	System error: failed to load schedule module
9320	Notification failure paths: failed or retry

When a Schedule Is Locked (8311)

Not every operation locks on the same statuses.

Operation	Rejected with 8311 when the schedule is
`PP` — process payment	Complete (`C`) or Suspended (`S`)
`AC` — attach card	Complete (`C`) or Suspended (`S`)
`PA` — add payment	Suspended (`S`) or Cancelled (`X`)
`SS` / `SR` / `SC`	Complete (`C`) or Cancelled (`X`)
`StartSession` with `SCHEDID`	Anything other than New (`N`), Ready (`R`) or Card Error (`E`)

PA will add a payment to a completed schedule, but the schedule stays Complete — and PP
then returns 8311 for that payment, so it will not be processed. Create a new schedule instead.

PP and AC are not blocked on a cancelled schedule, but every payment on one has been
stopped, so PP returns 8309 instead.