Sports players are "on the field." Technicians who work for a service company are "IN the field" when they leave the office to go work at clients' sites.
The players are on the field isn't accepted - why?
You should report this. ".. in the field", is not correct in English grammar
(Unless you are specifically talking about cricket - that most English of sports - in which case you can say that one side is "in the field", while the other side are batting. But that is just one specialised case!)
Same with baseball, "in the field" and "batting."
"The players...," including the definite article "the," is a correct translation and should not be marked wrong.