This declaration of $Computers collection....
$Computers = @(Import-CSV $Computers_Path -header "Name","MAC")
...will not work with this condition:
IF ($Computer.Result -ne "Successful")
The exception message explicitly states this, and where:
Property 'Result' cannot be found on this object. Make sure that it exists. \TestFunction.ps1:86 char:17
To resolve this, I would recommend initializing the property of Result, most likely like so:
$Computers = @(Import-CSV $Computers_Path -header "Name","MAC","Result")