I'm talking about the meat king mission. The guy on the start lays dead/unconscious. I take his clothes. Get into the main door. Go upstairs, take the girl's arm, kill the butcher with the wire and, after a bit of time, "The guards are looking for a suspicious bald person", then "your cover was blown."
I sneak around to change clothes for a waiter. I change, then go around for the second target, a guard spots me and bam.

No one has seen me killing the first target (The butcher), so there's no reason to suspect of a "bald person" who just came in, wander a bit and strangle a guy without anyone seeing it. Even worse, as then I change clothes and a guard just shoot me on sight without alerting anyone.

What I'm doing wrong?

The delivery boy wakes up at some point and alerts the door man. The clean way to avoid this is to close the trailer doors. Directly killing him also works of course.

Delivery boy..? Didn't notice that guy existed, lol. Gonna try it then, thanks!

EDIT: Yeah, didn't saw the little red button on the side of the truck to close the doors. Unconscious enemies awake after some nap minutes...

And also, mission completed as Silent Assassin. Yay!

The level intro cinematic makes it seem like he is killed with the meat hook so I'm sure you and I and countless other players did not realise right away he is the one who alerts all the guards before you can finish the mission. Congrats on the silent assassin rating, don't beat your head trying to figure out how to do it again on the next mission (Bjarkhov's bomb?) because even when you know how to do it there are so many things that need to play out perfectly. In particular, when the bomb explodes, you must get lucky with the physics engine so that no dead bodies are thrown in front of any live characters as this will count as "body found" *facepalm*