=MROUND(x,y) gives a #NUM! error if in if one of x or y is negative and
one positive - to prevent that use

=MAX(IF(AND($U11=0,$V11=0),MROUND(MAX($S11-$X11,0),4),IF($X11=0,MROUND(MAX($W11-$N11,0),4),MROUND($W11-$X11,4))))

....although is this the exact formula you're using? Because the MAX
function at the start is superfluous, this is sufficient

=IF(AND($U11=0,$V11=0),MROUND(MAX($S11-$X11,0),4),IF($X11=0,MROUND(MAX($W11-$N11,0),4),MROUND($W11-$X11,4)))


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