My phone battery maximum capacity is 91%. What should I do?

You don’t need to do anything except note the information. This function is still in beta testing and may not produce perfect accuracy but it thinks your battery has 91% storage capacity compared with design maximum. The next note says the battery can support peak loads, so there’s no problem there. Finally, Apple specification considers a battery has reached end of normal service life at 80% so yours has plenty in reserve. You wouldn’t consider the oil in your car needs replacing half way between services, but that is down to 50% of life remaining.