Your player is null, you likely didn’t get it’s reference properly. Since you are trying to access it using a unique name, did you make sure to right click on the player node, and select “access as unique name”? And did you also make sure it’s called Player exactly, and not player?
Before the code executes try putting a print line to display which variable is coming in null. Then you can figure out where your issue is.
Print(player)
Print(markA)
Like the previous person said, you’ve assigning a property to a target that’s invalid/null. This causes the crash. So find whats causing it to be invalid. Maybe it’s in the player script/ scene, maybe it’s not named correctly in the scene your running, or maybe it’s not even in the scene you’re running!