The issue seems to be that you own the Prologue and have then purchased the GOTY Upgrade, instead of the GOTY Edition. As you’ve said that the pop-up told you to buy the upgrade, we’ve double checked that set-up and it appears to be working as intended; meaning that players are pointed towards the GOTY Edition.
Have you at any other point had anything else related to HITMAN installed? We’re in the dark as to why the GOTY Upgrade was offered to you and not the GOTY Edition.
The GOTY Upgrade specifically has a note to avoid this from happening:
"You must own either ‘HITMAN - The Complete First Season’, ‘HITMAN - Upgrade Pack’ or have individually purchased all episodes from Season 1 to be able to use this item. Search for the other ‘HITMAN Game Of The Year Edition’ options if you do not own any of these items.
Right now, you won’t be able to play the locations from the game as you don’t own them. You can either purchase the GOTY Edition to unlock them or contact Microsoft about the options that you have from here. If we can help anymore, please update the thread.