"In the summer, it does not eat too much."
Translation:V létě příliš nejí.
V létě příliš mnoho nejí - why not příliš mnoho? It's literally too much too- příliš much - mnoho
So why is this not accepted?
It is a literal translation, but it is not something a native speaker would use. The national corpus does not show any such expression ("příliš mnoho nejí") while there are 20 příliš nejí, 141 moc nejí. Google books search also does not show much, just one similar example, which is however a literal translation from Latin made to illustrate the Latin text as closely as possible.
Otherwise "příliš mnoho" per se is very common so we may think about accepting it here anyway.