T&P valve is really tight, not able to remove it
OK, that warning label is telling you that the valve is protecting the appliance from both over-heating and over-pressure (combination). The part about removal from point of installation is a warning that relocating it to another part of the system will not provide adequate protection (back in the day they sometimes located a pressure-relief valve on a pipe outside the home). You can remove and replace it. Pull off that plastic cover so you can see the threaded part. Get a bigger wrench, at the factory they use a pipe compound that sets hard. Your tank is iron and unlikely to be damaged