Translation:Spies respect one another.
ghoqwI' is actually both singular and plural. The plural markers are not required in Klingon and almost any noun that does not have a plural marker could be plural. The sentence woch ghoqwI' might mean either "The spy is tall" or "The spies are tall". Usually context makes it pretty clear which. However, Duolingo presents sentences without context, so in a case like that either translation would be accpeted.
The -chuq suffix does require that there be plural subjects so it is necessary that the unmarked ghoqwI' actually be plural in this case and it cannot be singular. We could make it even more clear by using the suffix -pu', but grammatically, ghoqwI' must be plural even without the -pu'. We mostly use the plural suffixes in this course, but from time to time we intentionally leave it off to make sure that you are capable of understanding sentences that don't use it.