Learn Farsi: در این اردوی اجباری، آنها یک خطا هم تحمل نمی‌کنند. - In this compulsory camp, they do not tolerate even one mistake.

This is a common way to express emphatic scalar negation in Farsi. By combining یک (one), a noun (خطا, mistake), and هم (also/even) with a negative verb (تحمل نمی‌کنند, they do not tolerate), you get the meaning "not even one [noun]". Literally, the phrase reads as "they don't tolerate one mistake also."

Could I add حتی (hatta) to this sentence to emphasize "even"?

Yes! You could say آنها حتی یک خطا هم تحمل نمی‌کنند. Adding حتی right before the phrase makes the "not even" emphasis slightly stronger, but the یک ... هم structure combined with a negative verb is perfectly complete and natural on its own.

Why is there an "y" sound at the end of اردو (ordu) before the word اجباری (ejbāri)?

That is the ezāfe linking the noun (اردو, camp) to its adjective (اجباری, compulsory). Because اردو ends in the long vowel sound "u", the ezāfe is pronounced as a "y" sound and written with a ی (yielding اردوی) to connect the two words smoothly without a harsh vocal stop.

Does اردو (ordu) specifically mean a military camp?

اردو is a general word for "camp." It can refer to a sports training camp (اردوی ورزشی), a school trip (اردوی دانش‌آموزی), or a military camp. The adjective (compulsory/mandatory) is what gives the camp in this sentence its strict, demanding context.