If you can’t seem to unlock The Forty Thieves quest in Assassin’s Creed Mirage, then you can check out this troubleshooting guide from Seeking Tech.
1. It is important to note that The Forty Thieves is currently only available as a pre-order bonus for the game.
If you pre-order the game digitally, then the quest should be included with the initial download and installation.
If you bought a physical copy, then you should look for a The Forty Thieves leaflet inside the game box. Follow the instructions from this insert to redeem the code on the digital store of your platform.
If you did not pre-order Assassin’s Creed Mirage before launch, then you need to wait until Ubisoft makes The Forty Thieves quest available for purchase and/or give the pre-order bonus away through other methods.
2. If you are sure that you have redeemed and downloaded the pre-order bonus, then you should make sure that you are far enough in the main campaign.
The Forty Thieves won’t be activated until you complete the Branching Out quest report your progress to Roshan in Baghdad.
3. You should also make sure that you have the latest patch downloaded for Assassin’s Creed Mirage.
A new update may fix The Forty Thieves unlock issues.