Hi Dave,
Welcome to the guitar, and to GuitarTricks!

It is TOTALLY NORMAL to have trouble with the warm-up exercise! It is meant to be something you work towards over time. For now, just do what you can, and don't worry about the notes or strings you can't get to yet.

Just be sure to continue to challenge yourself to do just a little more each day, rather than settling on one level. For every person, it comes together at a different pace.

All that matters is that you use the exercise to get your fingers limbered up at the beginning of each practice session, and then move on to working on other things.

Have fun! All best wishes, Lisa