Hello,
I never thought I would write something like this about an Assassins Creed game. So apparently, after 2 days of intense searching, I discovered that the game cannot be finished if Sigurd does not return to England after the A Brother's Keeper mission. For Sigurd to do that, I found out that you have to make some decisions in his favor along the game (Leaving the materials in Norway or Giving Dag his axe), even if they are not always good. Not being able to see into the future and not knowing what decisions need to be made for the END THE GAME, I took the ones I felt at the time that are right, and now Sigurd remains in Norway, and the game, SURPRISINGLY, can no longer be terminated because of this. The father is not killed, but the rest of the order yes, all regions are finished except for Hamtunscire (which I CAN'T plead because of Sigurd's absence) and the settlement remains close to level 6, minus a SINGLE upgrade, and of course NOT being able to upgrade because I need to finish the story to do that. So after I struggled to find out this horrible thing, thank you Ubisoft. And I'm pretty sure that I'm not the only one in this situation.
You'll never see a penny from me again for an Assassins Creed game.

Hello. I don’t know if you solved this or not, but I encountered the exact same bug today, so I changed my graphics settings, after I heard of this on a forum. For example, I play on low, so I changed to medium, did the challenge and collected the treasure and then changed back to low. This workaround with the graphics settings worked for me in multiple occasions on Valhalla because apparently this Game has a lot of bugs like this (thanks Ubi so, in conclusion, if you ever encounter any kind of bugs like this, try changing the graphics. Sorry for my English