Dear Unchalee,
Good observation. The monthly sales in the SR report is calculated as difference between current YTD sales figures minus YTD sales figures of last month.
The negative EUR amount shown in the monthly column reflects the currency translation effect (THB EUR) from previous month to current month. Even with the same YTD sales figures in THB due to currency rate changes it can happen that you see in the report an EUR amount in the monthly column.