If this is the same as the English colloquial "big and strong" you should allow that, even if alto is directly translated to tall.
Why is altas e fortes not accepted?
It should be...
in another sentence altos means big , and this one means tall, they should fix it
grande = big
maior = bigger
alto = tall
mais alto = taller
If you would link to the sentence or say what the sentence was that used "big" for "alto" someone might be able to explain the discrepancy.
why isn´t "high" accepted? Thanks!
Because a person cannot be "high" (at least not legally). Only places are high.
"The tall skyscraper" but "the highest point of the plateau/cliff"
Or, figuratively, "the high point of our vacation/adventure"
What if a person is located in a high place? "We are high in the mountains"? Nos estamos altos nas montanhas?
Exactly! Very good. That would be «estar» and not «ser». :D
"Nós somos altas e fortes" should have been accepted