The volume computation in polymake is always done with respect to the ambient space. Your polytope lives inside the hyperplane H defined by the equation x1+...+x6=1, so it has at most dimension 5, and as such its six-dimensional volume is always 0.
What you probably want is its "relative volume", i.e., its volume considered as full dimensional point set inside the affine hyperplane H. There is no property implemented in polymake that would compute this directly. You can either transform your polytope into a full-dimensional one by applying a transformation that sends your polytope to R^5 (and take into account its determinant), or you artificially make the polytope full dimensional by, e.g., taking a pyramid. The latter approach is described in detail
in this forum post.
Best
Andreas