Translation:He often goes abroad on business.
So,"he goes often abroad on business" is wrong???
Yes, the word order is wrong.
Why is it ..naar het buitenland..? I thought in het buitenland meant 'abroad'.
If you'd use "zijn", you could use in. "Hij is vaak in het buitenland voor zaken." For the verb "gaan", we use naar. I would not translate abroad as "in het buitenland", but just as "buitenland". The preposition depends on the verb that you use.
"He often goes OVERSEAS on business." is wrong?
Yes, because buitenland simply means abroad without any indication if that means crossing a sea or not. I know some people always have to go overseas to go abroad, but that is a different issue.