This version of Unity is unsupported.
Method group is Obsolete

PrefabUtility.ReplacePrefab

Obsolete public static GameObject ReplacePrefab(GameObject go, Object targetPrefab);
Obsolete public static GameObject ReplacePrefab(GameObject go, Object targetPrefab, ReplacePrefabOptions replaceOptions);

Description

Replaces the targetPrefab with a copy of the game object hierarchy go.

Returns the Prefab game object after it has been created. If connectToPrefab is enabled go will be made an instance of the created Prefab.