I have received an error response on API during development. Tell me how to handle it.

Errors of http status: 4XX are mainly caused by merchants. We kindly ask merchants to conduct investigation and review.
However, the error of http status: 429 is caused by temporary server congestion. Please wait for a while and try again at a later time.
It is highly likely that errors of http status: 5XX are caused by PayPay. Contact us for inquiry.