Can I substitute amaretto syrup for amaretto liqueur?
no not at all. alcohol in recipes is generally there because certain flavor compounds are only soluble in alcohol. adding a half cup of syrup will make your recipe come out much much sweeter than it should. A better substitution would be probably around a teaspoon of almond extract and some neutral liquor like vodka. It would also be helpful if you include what recipe you are using or what you are attempting to make.