I've counted and checked but can't seem to find the problem.
\begin{flalign*}
u_0(x,\ t)\ =&\ x\left[t\ +\ \frac{t^3}{3}\right],\\
u_1(x,\ t)\ =&\ x\left[t\ -\ \frac{2t^5}{15}\ -\ \frac{t^7}{63}\ -\ \frac{t^{2-\alpha}}{\Gamma(3\ -\ \alpha)}\ -\ \frac{2t^{4-\alpha}}{\Gamma(5\ -\ \alpha)}\right],\\
u_3(x,\ t)\ =&\ x\left[t\ -\ \frac{t^3}{3}\ -\ \frac{2t^5}{15}\ +\ \frac{t^7}{45}\ +\ \frac{2t^9}{567}\ -\ \frac{4t^{11}}{2475}\ -\ \frac{4t^{13}}{12285}\ -\ \frac{t^{15}}{59535}\ -\ \frac{2t^{2-\alpha}}{\Gamma(3\ -\ \alpha)}\\
&+\ \frac{t^{3-2\alpha}}{\Gamma(4\ -\ 2\alpha)}\ +\ \left(\frac{2}{\Gamma(3\ -\ \alpha)}\ -\ \frac{2}{\Gamma(4\ -\ \alpha)}\right)\frac{t^{4-\alpha}}{(4\ -\ \alpha)}\\
&+\ \left(\frac{4}{\Gamma(5\ -\ \alpha)}\ +\ \frac{16}{\Gamma(6\ -\ \alpha)}\right)\ \frac{t^{6-\alpha}}{(6\ -\ \alpha)}\\
&+\ \left(\frac{80}{\Gamma(8\ -\ \alpha)}\ -\ \frac{4}{15\Gamma(3\ -\ \alpha)}\right)\frac{t^{8-\alpha}}{(8\ -\ \alpha)}\\
&-\ \left(\frac{8}{15\Gamma(5\ -\ \alpha)}\ +\ \frac{2}{63\Gamma(3\ -\ \alpha)}\right)\frac{t^{10-\alpha}}{(10\ -\ \alpha)}\ -\ \frac{4t^{12-\alpha}}{63(12\ -\ \alpha)\Gamma(5\ -\ \alpha)}\\
&+\ \left(\frac{2}{\Gamma(5\ -\ 2\alpha)}\ -\ \frac{1}{\Gamma(3\ -\ \alpha)^2}\right)\frac{t^{5-2\alpha}}{5\ -\ 2\alpha}\ -\ \frac{4t^{7-2\alpha}}{(7\ -\ 2\alpha)\Gamma(3\ -\ \alpha)\Gamma(5\ -\ \alpha)}\\
&-\ \frac{4t^{9-2\alpha}}{(9\ -\ 2\alpha)\Gamma(5\ -\ \alpha)^2}\right],\\
\vdots
\end{flalign*}

\left, but no\right. Things are out of sync. If it was your intent to split\leftand\rightacross lines of theflalign, that is not permitted. Instead use\bigg-style left/right arguments, instead of\leftand\right. See http://tex.stackexchange.com/questions/231696/error-while-equation-splitting-with-align-environment – Steven B. Segletes May 05 '15 at 19:22