POST request return error - Response verification failed

Im sending a POST req. with Postman. The req. body passes through, but the return gives me an error
Response verification failed: Certification values not found

With some digging it seems I have to create a custom certificate() which i should pass into headers.
Would anyone have a simple solution for this or just simple instructions?

I have no idea of Motoko, however, indeed, on mainnet, every HTTP response must include a certificate header to validate the response (wiki). One solution is to pre-compute the certificate when data is added to the canister’s memory, that’s what’s done in canister that host dapps such as in Juno, NNS dapp, Internet Identity or the DFX assets canister (doc, doc or doc). If that’s not feasible, if you need a dynamic answer, you could respond to the HTTP request using an update call (doc). Hope that helps.