Errrrr to adjust the valves you remove the "caps" (they are the same part as drain cap both CB or CG)
You pulled off the totally wrong part etc etc.
Here's a link if this helps with a basic tool set for such a job...
https://www.ebay.com/itm/303381509504
If I recall the valve train cap also has HEAD NUTS - you will want to re-torque etc and hopefully no other issues do to head being released.
While it's apart you could add some roller rocker in there, be a little quieter n smoother action etc