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.
-- Gianluca Sartori