Please turn of the heating appliance, then release the pressure to 0 bar on the gauge on the left top corner, then close all valves you can find, it is not necessary to close the valves, but it can
the start up procedure. Then go ahead and reseal. Don't forget that copper gaskets can be really useful and robust.
After repair, please follow manufacturers air bleed procedure before starting the system. There is a chance it is just about bleed air through a safety valve, the one you can easily release the pressure with aswell. This item is not visible on the picture.