There is a method and it is simpler than the fabric names suggest. Ignore the fibre and match the behaviour.
The envelope lists poplin, chambray and linen — three different fibres that behave the same way: mid-weight, crisp, holds a fold, does not cling. That shared behaviour is what the pattern was drafted around, not the fibre content. So the question is not "is viscose twill allowed", it is "does this behave like those".
Three tests, all of which take a minute:
The drape test. Hang a length over your hand or the edge of a table. Crisp fabrics stand out and make a wide, shallow curve. Drapey ones fall almost straight down in narrow folds. Compare against a shirt you own made of poplin. Yours will fall much further, and that tells you how far off you are.
The scrunch test. Grab a handful and let go. Crisp fabrics hold creases and spring back into a shape; drapey ones fall out flat.
The pin-to-yourself test. Pin a piece to your shoulder and look in a mirror. This is the one that actually answers the question, because it shows you what the garment will do rather than what the fabric does.
Your viscose twill will make a real garment. It will be a softer, closer, more fluid dress than the one on the envelope — a different design, not a failure.