This error occurs when the publisher of the webservice is using SSL (https), but using a certificate which is not signed by a public known authority (CA). If you want to trust this publisher, you can manually add this certificate.
First download it: for example using your browser: browse the site, click the lock in the address bar and click details and Export to file (CER or CRT-file).
Next import it. Locally in your Mendix Modeler: Project Settings > Certificates > Import. And in the Mendix cloud: go to your environment details > Network tab > Outgoing Connections Certificates > Add Authority.
please, can you provide more information about how you have been implemented your code?