Pressure relief valve stuck - leave alone or force it?
You should remove and replace the valve.  If you cannot open it using the lever chances are good that it also will not function properly when needed.  So you have a dangerous condition.
It looks like you have limited space to work in so you may need to remove the downpipe from the valve before you can remove it.  But a new one is cheap insurance vs. a thermostat failure leading to a rupture or worse.