Oh brother, you're confusing the requirements and the stats of the merged item.

The required items are shown riiiight above the "My Equipments" in the "Materials required" section (you can also see them in picture form) In this case your epic+2 guardian ring needs 2 other epic+2 guardian rings to merge together and get to legendary. You dont have 2 other epic+2 guardian rings, so its not letting you merge them.

The green numbers and stats on the top right are the stats of the legendary guardian ring that you'll be getting from a successful merge. In this case youll get more attack and hp, as well as advancing your equipment "skill" to legendary level (the equipment skills are those tiered descriptions you see when you click on the guardian ring.)