During the main story mission “Search and Destroy,” go to the apartment building with Takemura
After the explosion and ambush, do not leave the building immediately
Follow Takemura through the hole in the wall and into the next area
When Takemura is separated from you, keep moving through the building instead of escaping
Find the stairs and go upstairs
Fight through the enemies and reach the room where Takemura is trapped
Open the door and help him
Leave the building with Takemura
Do not ignore the objective to rescue him
If you already left the building, reload an earlier save and repeat the mission properly