I was running into this error when doing some development work using Postman on my Windows workstation:

authorizationError: “UNABLE_TO_VERIFY_LEAF_SIGNATURE”

PowerShell and an actual browser was not running into this issue. These applications leverage local certificate store to validate a certificate trust. Postman does not do this. One simply needs to get the root certificate for the PKI, convert it to a pem file and then configure it in Postman.

Converting a cer to pem: https://knowledge.digicert.com/solution/SO26449.html

Managing certs in Postman: https://blog.postman.com/encryption-ssl-tls-and-managing-your-certificates-in-postman/

I configured the pem file in Postman certificate management and the error went away!