Insert Conv. LCY. Rndg. Lines no response to click

Hi, my LCY is USD, this vendor is SGD and the bill was SGD, so now is trying to pay by USD Bank, I don’t understand what’s wrong with the entry but apparently, when I clicked on the "Insert Conv. LCY. Rndg. Lines, nothing happens.

To be added, I have setup the G/L Account for Rounding posting for both Debit and Credit in SGD.