The error message “isnt declared in the current class” means the type of node you have chosen doesnt have this property. A kinematic body would have this property for instance but a basic node wouldnt. What kind of node is your bullet made of?
To fix your error, you need to change node’s script owner to Node2D or it’s descendants (and add extends Node2D as first line of your script) class. get_global_position is defined in Node2D class. Based on your script, your node is extended from Reference (if your script has no extends keyword, then your script extends Reference).