I believe you can’t use functions like this, they are local to Blueprint.
If you want to make some sort of utility functions, which can be used by every BP — create Blueprint Macro Library and add new Macros. It has almost same functionality as a functions, can have local variables and can be used everywhere in your project. As an example - ForLoop and ForEachLoop are macros.
If you have a set of different blueprints, for example - Sword, Axe and Hammer, and want to have “Swing” function for them, I recommend you to create a Blueprint Interface “Weapon” and add functions like “Swing”, “Parry” and etc, which are common for weapon classes. Then implement this Interface in sword/axe/hammer blueprint options and you will see Interface functions inside your basic Blueprint. However, this functions will be empty by default and you can specify what exactly function doing in particular Blueprint.