It does not affect safety.

[notlangley]

Transaero spokesman Konstantin Tyurkin said all that was missing from the wing was the cover to a hatch workers use to check the left engine. It was not immediately clear who had misplaced the hefty piece of equipment or why.
"This was a technical element. Its absence doesn't prevent takeoff," said Tyurkin. "It does not affect safety."

[KBPsen]

The man is quite right, it does not affect safety. Up to 4 of these access doors may be missing on each pylon.

[Sunnyjohn]

The man is quite right, it does not affect safety. Up to 4 of these access doors may be missing on each pylon.

. . . unless it happens to fall on you on its way down.