If you live in the US, I agree that you must use American English.
But even us people that live outside of the US/UK/related countries often got errors marked, because we used the wrong regional variant... In Europe, British English is used as a reference point, but I had a similar problem as GP.
But even us people that live outside of the US/UK/related countries often got errors marked, because we used the wrong regional variant... In Europe, British English is used as a reference point, but I had a similar problem as GP.