DataGroup (string & value) tolower

Apologies... I read this at least two ways...

Do you mean you want to return the value (strCountryAuthorization in your code) and then convert it to lower case?

Use

[string tolower $variable]
Returns string with all the letters converted from upper to lower case.

This should do what you're asking for. Simply use the [string tolower $variable] TCL command to convert the string. e.g.

when ACCESS_POLICY_AGENT_EVENT { 
  if { [set strCountryAuthorization [class match -value $strCountryCode contains Blocked_Countries_dg]] ne "" } {
   } else {
     set strCountryAuthorization "allowed" 
   } 

    Terminate Session if Country is Blocked 
   if {[string tolower $strCountryAuthorization] eq "blocked"} {

        Kill Session
       ACCESS::session remove
   }
}

I'm not sure if you use strCountryAuthorization elsewhere if it's set to 'allowed'. If not, then that 'else' setting it to allowed is wasted CPU cycles.