It looks like dbo.region_scorecard is a table or a view.

In that case, you need to provide a predicate to correlate the table dbo.current_year with it:

SELECT ISNULL(dbo.merica.Country, dbo.current_year.[Mega Region]) AS TEST

FROM dbo.current_year

CROSS APPLY (

SELECT ISNULL(dbo.region_scorecard.[GAM Customer for Scorecard], TEST)

FROM dbo.region_scorecard

WHERE dbo.region_scorecard.PrimaryKey = dbo.current_year.SomeForeignKey

) AS [Regions/GAM]

LEFT OUTER JOIN dbo.merica

ON dbo.current_year.[Mega Region] = dbo.merica.Country

I can't see your database definition, so it's just guesses. If you want precise answers, post more information.