If you encounter a "An authentication tab will open. If it does not, you can click on the following link to open it. here" error like the above screenshot after clicking on SightCall widget in attempt to connect to SightCall platform to start a call, then it means you logged in to Salesforce by impersonating (Log In As) a different Salesforce account.
At the moment, SightCall for Salesforce package doesn't support impersonation. If you want to test using SightCall within Salesforce, you will need to login using the actual Salesforce account without impersonation.