PaygateDeveloper documentation
400idempotency_key_too_longplatform

Idempotency-Key is longer than 255 characters

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

The header is capped at 255 characters.

Why the API refuses

Idempotency-Key must be at most 255 characters

What to do

Use a UUID. A key built by concatenating a JSON payload is both too long and a privacy problem in your logs.