On my system I get the result {Null}
. That's because Do executes but does not give an output.
This seems to work:
zero1 = x /. {ToRules[Roots[x^2 - 4 == 0, x]]};
xval[k_] := zero1[[k]]
yval = 0
Table[{xval[b], yval}, {b, Exponent[x^2, x]}]
However, I would rather achieve the goal this way:
{Re[x], Im[x]} /. NSolve[x^2 - 4 == 0, x]