As of July-16-2018 , CA Agile Central including its Excel add-in no longer support TLS 1.0 or TLS 1.1.
We support TLS 1.2
When using SSO then your Excel add-in is issuing a HTTP call to your SSO system which in-turn will authenticate your add-in to Agile Central. This is using your default browser's security settings where the TLS versions are specified.
You need to make certain you enable your default web browser to TLS 1.2. This procedure is explained for every browser here:https://knowledge.digicert.com/generalinformation/INFO3299.html
To be safer, you may want to enable all your browsers to TLS 1.2 since you'll need it if you use any of them for Agile Central (or the Excel add-in).