When accepting a pickup task, always make sure that the task is really yours. If you see the name of the restaurants and details at the bottom panel in the main screen, then the task is yours.
Sometimes, when you accept a pickup task, you get an error message saying, "You have no permission to start the task".
In this case, the address assignment algorithm has already assigned the task to someone else, but it has not been updated on your device yet (it needs a couple of seconds).
The root of the issue is locking your phone's screen right after accepting the task and going to the restaurant (the information that it's no longer your task won't get to you).
After accepting a pickup task, always wait 1-2 seconds to make sure you really have the task.