It'll do you good to have a rest.休息一下对你会有好处的.我认为应是It'll do g

do sb good该结构表示“对某人有益或有好处”,其中的 good 为不可数名词。如：Exercise will do you good。运动对你会有好处。It'll do you good to have a rest。休息一下会对你有好处。这个结构是没有问题的。