The GetState action reads a value to a specific Flow State field. For this action to work, the Flow needs to have a State Field with this exact name.

GetState(
  name= "<State Field name>",
)

Example

{{SetState(name= "my_state_field", value= "My Very Important Value")}}
{{set(name="my_state_field_value", value=GetState(name="my_state_field"))}}    
{{SendMessage(message=my_state_field_value)}}
My Very Important Value