er oder es

In lesson 9.4 Mathias is asked to describe his suitcase he begins with es ist is there a reason why this is not er ist. I know this is a minor point but it is in my head.   Michael

There is quite a lot of discussion of this very point here:
https://members.rocketlanguages.com/members/forum/german-grammar/lesson-94

I expect you missed it because the thread title is not very specific.

Hi michael-m301 und sfpugh,

thank you for posting the link sfpugh! 
Have a look at that discussion and if you have any additional questions don’t hesitate to ask. 

Viele Grüße 
Julia