"Es cierto."

You might say "es la verdad"

I'm thinking it might be European Spanish/Latin American Spanish

And you were correct, etymologically, whatever DL contends. I did the same and shall report it.

I agree, but it marks it wrong

verdad = truth cierto = true ..... I think

To quote EleanorD1 below, "Sure" in its usual sense is a state of mind, not a state of affairs. It is possible to use "sure" in this sentence, but it's kind of unusual or archaic and it means "reliable" more than "true". Like "God's justice is sure".

I also think of sure as being secure or certain. Not just a state of mind, but as in a mechanical sense. The bolt is sure, as in a sure hold.

"It's" means "It is", so "It's is right" will be like saying "It is is right."

Grammatically, you would say either "it is right" or "it's right." The meanings of "it is right" and "it is certain" are different.

En ingles, "Its is right" es como decir "Esta es correcto" en español. In English, "It's" means "that thing is" or "That has" or "It is" if used as a contraction. "Its" means "That thing is owned by that other thing". http://www.its-not-its.info/ What is your native language? Someone here might be able to translate more accurately.