Answer
The step $=1+(-1+1)+(-1+1)+(-1+1)+...$ is incorrect since addition is only associative for finitely many terms, not for infinitely many terms.
Work Step by Step
$0=0+0+0+...$
$=(1-1)+(1-1)+(1-1)+(1-1)+...$
$=1-1+1-1+1-1+...$
$=1+(-1+1)+(-1+1)+(-1+1)+...$ (1)
$=1+0+0+0...=1$
Hence, the step $=1+(-1+1)+(-1+1)+(-1+1)+...$ is incorrect since addition is only associative for finitely many terms, not for infinitely many terms.
