[tex]f(x)[/tex] has to be differentiable on [-1, 1] for the theorem to hold. This is not the case because the derivative doesn't exist at [tex]x=0[/tex].
[tex]f(x)=\sqrt{\left(2-x^{2/3}\right)^3}=\left(2-x^{2/3}\right)^{3/2}[/tex]
[tex]\implies f'(x)=\dfrac32\left(2-x^{2/3}\right)^{1/2}\left(-\dfrac23x^{-1/3}\right)=-\dfrac{\left(2-x^{2/3}\right)^{1/2}}{x^{1/3}}[/tex]
but clearly [tex]f'(0)[/tex] is undefined.