The issue is that your visitors’ browsers are unable to properly identify who issued your certificate. First, confirm that your visitors are not seeing an incorrect or outdated certificate. Once you have made sure that your visitors are seeing the correct certificate, the issue is most likely solved by installing the intermediate certificates.
Below are the links that you can use to download your intermediate certificate from the vendor website:
- https://knowledge.digicert.com/generalinformation/INFO4331.html
- https://knowledge.geotrust.com/support/knowledge-base/index?page=content&id=AR1421
- https://search.thawte.com/support/ssl-digital-certificates/index?page=content&id=AR1384
- https://knowledge.rapidssl.com/support/ssl-certificate-support/index?page=content&id=AR1548
- https://support.comodo.com/index.php?/Default/Knowledgebase/List/Index/108/sha-2
