The first level spell Bane allows you to curse a creature so that each saving throw or attack roll is reduced by 1d4 for the duration:
Whenever a target [..] makes an attack roll or saving throw before the spell ends, the target must roll a d4 and subtract the number rolled from the attack roll or saving throw. (PHB 216)
How does this interact with an attack roll of natural 20?
I can see two possible interpretations:
Bane is stronger: A target under the effect of bane cannot make critical attacks, because 20 – 1d4 will always be < 20.
Critical is stronger: A target under the effect of bane can make crits, since crits occur on a natural 20, and the bane malus is applied after determining crit.
A good answer would include relevant rules text to show how this should be handled.