Why is battery maximum capacity disabled?

Sometimes a generic battery will not allow for battery health or capacity. Depending on where you got it, it might just be a bad battery. Personally, I would get rid of it and buy one from iFixit. I have bought several, and they are top notch quality. They may be more expensive than ones from eBay, but they will last so much longer. iFixit also has a warranty on them so you don’t have to worry about it.