EDIT: I had to deposit the targets body in the trunk of the car. Problem solved.


I killed the guy and finished the mission "Doing humanity a favour." This mission is to kill a boxer who killed his boxing partner, with a punch, and dumped the body.

El Captain sent a car to collect me for some reason. I Can't interact with car. I can't get inside, I can't blow it up, nothing.

Hello,
Try reloading a save from before you completed the mission and attempt it again. This can sometimes clear up any glitches. If you’re playing on PC, verify the integrity of your game files through the game launcher maximtimeclock (Steam, GOG, etc.). This can fix any corrupted files that might be causing the issue.

Sent a car to collect you? Fixers only send cars to your job if you've 'knocked out' your target, they want you to carry the body to the car and put the unconcious target in the trunk. If you kill a target, they don't send a car.