The expression [[a,b,c],[d,e,f],[g,h,i]] is not an array in MuPAD. It is a "list of lists." I'm guessing that's not what you're after. Lists are commonly used to initialize arrays and matrices and other objects (more here). For these examples, either arrays or matrices will work, but note that these two...

The reason this works with MuPAD's mtaylor syms x y; feval(symengine,'mtaylor',exp(x^2 - y), x, 4) % [x] is fine too and this doesn't syms x y; feval(symengine,'mtaylor',exp(x^2 - y), [x, y], 4) is that the [x, y] argument is seen as a single symbolic vector argument/variable rather than a list...