a Javascript snippet is not a function that will return a value, in your snippet, comment back in line 22 from the second screenshot and . comment out lines 12, 13 and 14 (its a function with no call to it).

myVar is text so -1 wont substract a 1, we need to turn it into an integer. for example context.variableMaager.setValue("myNLvar", parseInt(myVar)-1) where myNLvar is the neoload variable in your request