This seems to be an issue with trying to use a standard macro in a widget inside a project that hasn’t used that standard macro anywhere else yet.
Bizarrely, just creating a BP class and adding one of the macros from the Standard Library (For Each Loop, For Loop, Is Valid, etc.) to its graph will make those macros also available inside user widgets.
So: quick fix, make a Blueprint class, add one of those macros to it, compile, and then switch back to your widget.