You need to check isempty(z) before you try to access the results.
That is, you are assuming that dsolve() is able to find a solution, but it is not able to find a solution.
There is no explicit solution for your equations. There are some families of solutions. Some members of the families involving picking arbitrary y2(t) or y5(t) with the other functions becoming various complicated results, some of which are closed form and the others of which can even end up being differential equations that might or might not have closed forms (but probably not) depending on the arbitrary choose of functions.
