Помимо отсутствующего return
statememt, вы можете уменьшить массив и принять измененную строку в качестве аккумулятора.
var unitsLong = ['tablespoons', 'tablespoon', 'ounces', 'ounce', 'teaspoons', 'teaspoon', 'cups', 'pounds'],
unitsShort = ['tbsp', 'tbsp', 'oz', 'oz', 'tsp', 'tsp', 'cup', 'pound'],
ing = ["8 ounces cream cheese, softened", "1/4 teaspoon garlic powder", " teaspoon dried oregano", " teaspoon dried parsley", " teaspoon dried basil", "1 cups shredded mozzarella cheese", "1 cups parmesan cheese", "1 cups pizza sauce", "1/4 cups chopped green bell pepper", "1/4 cups chopped onion", "2 ounces sliced pepperoni"],
newIng = ing.map(el => unitsLong.reduce((s, unit, i) => s.replace(unit, unitsShort[i]), el));
console.log(newIng);