Hello Sir,
I know the problem you are facing with Photon PUN2 matchmaking issue. I suffered the same issue when I was in my early stages of PUN2 learning.
The problem is currently your script is trying to create or join room without ensuring it is connected to master client server while being on Game server only.
I can surely remove that error and also explain you what is happening in the background in detail in c# script. You can always expect a friendly and a polite conversation from my side. looking forward to your reply. Thank you!