JavaScript - 获取用户输入并在计算中使用以验证值

I have a modal that allows users to 'refuel' by filling out a form where they can input the amount of fuel to add, however the maximum fuel amount is 180 litres, so if the existing fuel is 170 litres (the existing fuel value will be retrieved from a database), and the user tries to add 20 litres, it should produce an error. I've got some code already, but it's not producing the error. If anyone could point out the issue it would be greatly appreciated. I have a modal that allows users to 'refuel' by