What book are you reading? I've never heard of a "Calvino", and I am assuming "Crasso" and "Varo" refer to Crassus and Varus. Crassus was an important figure of the Late Republic, while Varus was the Roman commander responsible (in part) for the defeat at Teutoburg Forest in 9 A.D. Neither feature in any scholarly works I've ever read on the collapse/end of the Western Empire in 476.

As u/HatMaster12 suggests, you should be extremely, extremely cautious/sceptical of anyone who tries to suggest that an event or decision made in the 1st century B.C. somehow caused something that happened more than 500 years later. Or up to 1500 years later, depending on when you think "the Roman empire" finished falling.

So the answer to your question about how responsible these people were for the fall of Rome, quite unequivocally, is: "They weren't."

In some more traditional histories, especially in the 19th century, people had this idea that if an empire stopped expanding and conquering new territories, this meant it was starting to decline. Presumably your book still upholds this view, and thinks that Varus' defeat against the Germans and Crassus' defeat against the Partians mark the end of Rome's expansion and thus the beginning of its decline.

Like I said above, this view really doesn't make a lot of sense. The premise doesn't make sense, and Rome's expansion didn't even stop with these defeats, with emperors like Claudius and Trajan continuing to add territories to Rome's dominion.

If you want to know what historians have to say on why the (western) Roman empire did fall (or whether it's even accurate to talk about it "falling" at all) you might want to read this post of mine and the (lengthy) discussion that follows.