taumin is not complex, so imag(taumin) is zero everywhere. There is no contour to create.
You initialize
taumin=ones(nu,nxi);
which is not complex.
You set
taumin(i,j)=min(tausolR);
where
tausolR=imag(tausol_found);
You extract the imaginary component from tausol_found so the result is a strictly real number, and real numbers always have an all-zero imaginary component.
If the task is to include 0 in the level list then use
LL = union(0, min(min(taumin)):10000:max(max(taumin)));
contourf(U,Xi,taumin,LL);
Beware though, that min(min(taumin)) is -4.32538271227367e-07 and max(max(taumin)) is 1, so min(min(taumin)):10000:max(max(taumin)) is -4.32538271227367e-07:10000:1 which is just -4.32538271227367e-07
