Error message :PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target HTTPS server Certificate Issuer is "COMODO RSA Domain Validation Secure Server CA" with Signature hash algorithm Name "Sha256".
Here is the sample code
HttpRequest req = new HttpRequest();
req.setEndpoint(<HTTPS server>);
req.setMethod('POST');
req.setHeader('Content-type','application/json');
HTTPResponse res = http.send(req);
System.debug(res.getBody());