422payments.invalid_reason_codepayments

The refund reason_code is not one of the reason codes

Returned by the implementation; no specification rule declares it yet.

A refund carries a reason_code beside its free-text reason. The code must be one of these integers:

Code	Meaning
`100`	The payer was charged twice
`101`	The payment was fraudulent
`102`	The payer asked for the money back
`103`	The goods or service were not delivered
`104`	Defective, or not as described
`105`	The order was canceled
`106`	The wrong amount was charged
`999`	Any other reason, described in `reason`

Why the API refuses

reason_code is not one of the refund reason codes

Send one of the codes above, or leave reason_code out: a request that names no code is 999, so a caller that only sends reason keeps working.
For 999, put the detail in reason (up to 255 characters); it is free text, so keep personal data out of it.
Do not send 900. It is reserved for the sample data that POST /test/seed creates.