i see you link game manager node here, i guess your coin is instantiated and add_child from editor in a main scene?
your main script should know where the score_board scene node is, and when coin is collected, you just send signal telling a coin is collected to Main Script, so the Main Script can tell score_board to update the label points
Scoreboard is probably best used through an Autoload, otherwise if your coins are placed manually they would each need to be connected or given a reference to the scoreboard. If they are made dynamically/spawned in that is less of an issue.
Oh an the unique identifier only works for nodes within the scene of said script, so %GameManagar will not give you a game manager since it does not exist in coin. Probably autoload that as well
do you have a main script here? it doesnt look like the game manager is the main script now, the Main Script should hold all the essential node in a variable or two, then use the variable to tell the node to do something when receiving signals to do something
Autoloaded .tscn files will also allow access to it’s scripted variables and functions. Assuming the script is on the root node, which it is for your scoreboard.