404
checkout.session_not_foundcheckoutNo such checkout session
Returned by the implementation; no specification rule declares it yet.
The session token does not name a live checkout session. Sessions are short-lived: an expired one is indistinguishable from one that never existed, deliberately, since the token is a bearer capability.
Why the API refuses
checkout session not found
What to do
Start a new checkout. If a payer reaches this from a bookmarked URL, treat it as an expired basket and send them back to your own checkout entry point rather than showing an error page they can only reload.