If a throw is made to a base and the man covering the base is not there allowing the ball to go into the outfield who is charged with the error?

The error will be on the person who was to cover the based. this is as long as the person who was to cover the base could have gotten the ball.

If in fact there is nobody there to receive the ball, then it would be ruled a throwing error.

Offical baseball rules section 10.13(e) The Official Scorer section.An error shall be charged against any fielder whose failure to stop, or try to stop, an accurately thrown ball that permits a runner to advance, providing there was occasion for the throw. If such throw be made to second base, the scorer shall determine whether it was the duty of the second baseman or the shortstop to stop the ball, and an error shall be charged to the negligent player. Note: If in the scorer's judgment there was no occasion for the throw, an error whall be charged to the fielder who threw the ball.