Can I not say ''syv''? It reported it as a mistake
"Jeg spiste syv grønnsaker i går." is also correct.
Grønnsaker is actually countable?
Yes and no (Here yes). It depends on the context, but why would you think it's not? It means literally "green things" and last I checked things were countable. Although in some instances it (grønnsaker) isn't countable so beware.