And why not "In September the autumn starts"?
Or " the fall starts in September"?
why not...the fall begins in September
"Fall" is american. " autumn" is English. It should be accepted as well.
If you ask for the answer to be in English please use "Autumn", "Fall" is not an English word in this context. Otherwise you should also accept "Hydref" (Welsh but UK) as correct.
Why is not correct "The Autumn starts in September".
Is "Autumn" wrong because of the spelling?
English normally drops "the" on general statements.
Not always, perhaps in spoken English but in written English both are absolutely acceptable, it is especially natural if fall is substituted for autumn (as used in British English).
I prefer to use "autumn" because it is more common in countries where people speak English.
So it is normal in US English? I don't think it is normal in UK English.
From my region of the UK it's absolutely normal