Subquery returned more than 1 value. My WHERE clause needs to remove SELECT statements
I found the solution.

I replaced the = (equal signs) to IN. 

The WHERE clause now looks like this:

 WHERE 
    (END_DATE IS NULL OR END_DATE &gt; @REPORT_DATE)
    AND 
    REPORT_SPOOL.FUND_ID IN (SELECT FUND_ID FROM FUND_HLD WHERE [DATE] = @REPORT_DATE)
    AND
    REPORT_SPOOL.FUND_ID IN (SELECT FUND_ID FROM FUND_TRD_LINE_VIEW WHERE [DATE] &gt;= (SELECT DATEFROMPARTS(YEAR(@REPORT_DATE),MONTH(@REPORT_DATE),1)) AND [DATE] &lt;= @REPORT_DATE)