Hey there @willumsen1999! I apologize for the late response.
I'm also sorry to hear that you haven't received your 15th Order Medallion. At this point in your game, have you also defeated The Father? If not, go ahead and defeat him and see if he drops it.
If you have already defeated The Father, perhaps you can reload an earlier save game and fight him again if you have one available. If you have a save file where there are multiple Order members left to kill, I would recommend reloading that one because this glitch can sometimes cause players to only rece...