There are a few different intermediate certificate errors that you can get with the DigiCert® SSL Installation Diagnostics Tool. First, the generic "The server is not sending all required intermediate certificates." You can also get a "Your server is sending too many intermediate certificates" error.
See the below links for instructions on replacing the intermediate certificate on your server by platform. Alternatively, you can reissue and reinstall your SSL Certificate to fix the issue.
If you have any additional questions please contact DigiCert Support.