First you need to join your two tables in the load script, by adding the lines below in bold brown.
Give that a go & if needed post the revised qvw and ask for suggestions.
Monthname( InvoiceDate ) as [YearMonth] ,
(ooxml, embedded labels, table is [Sales Actual]);
monthname ( date ( date# ( text (num ( Month , '00' )) & Year , 'MMYYYY') ) ) as [YearMonth] ,
(ooxml, embedded labels, table is Forecast);
Best Regards, Bill