Hi,
The specified input offset voltage is 3mV typ and 7mV max.
Multiplied with a gain of 21 you get an output offset voltage of 63mV typ and 147mV max.
So with your 20mV you are well within specification.
(Still ignoring offset current, bias current and all offset drifts)
You may compensate by adding subtracting a fixed DC voltage.
Often done by a potentiometer and two resistors at +Vin.
This may improve static offset, but can't improve drift. Additionally it adds offset caused by power supply fluctuation.
If you are interested in better offset performance: look for Opamps with better offset specification.
Klaus