Rashi in d.h. ve-ayn osin, says that when Rebbi Eliezer says you can't make a succah on chol hamoed this refers to someone who didn't sit in a succah on yom tov rishon.
What is the rationale. If as the gemarah goes on to say, you need a succah fit for seven days, what does it matter what he did on yom tov rishon?
Rashi's intention is to explain that even if one did not sit in the Sukah on the first day, one may not make a Sukah on Chol ha'Mo'ed.
(With this, Rashi is answering a problem in the words of Rebbi Eliezer. Since Rebbi Eliezer already said that one may not go from one Sukah to another, why does he have to add that one may not make a Sukah on Chol ha'Mo'ed? If he makes a new Sukah on Chol ha'Mo'ed, that means he is leaving one Sukah and going into the newly built one on Chol ha'Mo'ed! Therefore, Rashi explains that even if he did not sit in a Sukah on the first day, he may not make a Sukah on Chol ha'Mo'ed.)
M. Kornfeld