'plus de' or 'plus des'?

 Bonjour Marian,

You are right that ‘des’ should be ‘de’. When expressing ‘more + [sth]’, you use ‘plus de’ + plural noun (if countable) or + singular noun (if abstract / uncountable)

See link here: comparative

However, the aim of the lesson is to use 'servir' and its derivatives accordingly. In this instance, you don’t need ‘plus’ as ‘ressers’ already implies being given ‘more’, aka a second helping, as per the definition of ‘resservir’ explained in the lesson:

resservir - to serve up again / to give a second helping
Therefore ‘des’ applies in this instance.

Do I serve you more potatoes? = Je vous ressers plus des pommes de terres ?

You could say something as below (although it is not part of this particular lesson):

Je vous sers davantage de pommes de terre ? = Do I serve you more potatoes?

I hope this is helpful.

Bonne journée !