case statement sub query need to return multiple values
This should work:

select  

CASE WHEN LTRIM(RTRIM([Order].ClientProject_Id))!=''
    THEN StationeryClientProject.ImpFile
ELSE Stationery.ImpFile
END AS ImpFile

FROM [Order]
    LEFT OUTER JOIN ClientProject ON   ClientProject.ClientProject_Id = [Order].ClientProject_Id
    LEFT OUTER JOIN StationeryClientProject ON StationeryClientProject.ClientProject_Id = [Order].ClientProject_Id 
    LEFT OUTER JOIN Stationery ON Stationery.Stationery_Id = [Order].Stationery_Id


The error message is caused by multiple rows being returned inside the case statement.  Since you are already returning the required rows in your FROM clause, you can simply reference the required columns in the case statement.