Creating the unreal skeleton imposes conditions that require a specific metarig. Posing the metarig doesn’t just update the pose, it also modifies the metarig to be different from the default.
The script is intended to add the Epic Skeleton to blank characters or characters with humanoid skeletons (not full rigs) like imported mannequin or marketplace characters. It was not designed to be used with random metarigs or characters already rigged with rigify without using the script.