That line of script is working for me with El Capitan. Which app are you running the script from? I wonder if it doesn’t have access to read the dictionary file in SpamSieve.app. If so, the raw syntax might work:
set isSpam to «event mtSSisSp» given «class Mess»:theSource
It’s in a “tell app SpamSieve” block within a “tell app Mail” block, but I get the error in just a plain “tell app SpamSieve” block too:
tell app "SpamSieve"
set isSpam to looks like spam message "hello"
end tell
The application is a command-line Swift application that uses NSAppleScript, but I get the error in AppleScript Editor too. Using the raw Apple Event syntax works though.