This error occurs when the proxy installer is not able to communicate with the Recast Management Server.
In order to see what is causing the issue, run the following command in Powershell invoke-restmethod https://<FQDN_OF_SERVER>:<Port_of_server>/api/action/test -Method post
The command returns a specific error that will assist in troubleshooting the proxy error.
A notable example returned from the Powershell command:
Could not establish trust relationship for the SLS/TLS secure channel - In order for the Proxy installer to connect with the Recast Management Server, you must export the public key that was used for the Recast Management Server and import it into the Trusted Root Certificate Authorities Store on the computer on which you are installing Recast Proxy.