PS C:\windows\system32> powershell -command "& { . C:\Users\user\Desktop\CasperFileVault.ps1; CasperFVault }"
Error Grabbing FileVault Key - The underlying connection was closed: Could not establish trust relationship for the SSL/TLS secure channel..

I was testing this in my environment, and found that it would return "No FileVault Key" every time. I'm not sure how it is supposed to scrape the key value when the page needs an action to occur (javascript:retrieveFV2Key()) in order for the value to be exposed on the page?

There appears to be a typo in line 9. As it reads now it is:
$auth = Invoke-WebRequest -SessionVariable Sessions -UseBasicParsing -Uri "$serverAddress/?failover'" -Body $data -Method "Post" -ErrorAction Stop

For this to work I had to change it to:
$auth = Invoke-WebRequest -SessionVariable Sessions -UseBasicParsing -Uri "$serverAddress/?failover" -Body $data -Method "Post" -ErrorAction Stop

Note the absence of the single quote after failover. Without this correction, mine kept trying to send me to the SSO login, but with it I was able to retrieve the FV key.

I noticed something else as well. If using a Jamf-hosted cloud instance for your JSS, port 8443 does not work, but port 443 does. You may want to set this as a separate variable or note it with a comment.