Exception:

System.CalloutException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target

Resolution:

This certification exception in Salesforce occurs when the endpoint has certificate issue.

Reach out to the Endpoint URL supporting team to resolve the Certificate issue since the issue is from Certificate configuration. Once the certificate issue is resolved, wait for sometime and test it.

There are free SSL checkers websites. You can utilize them if your employer/organization allows using it to check the certificate issues.