The following formula seems to do what you describe.
=FILTER($J$26:$J$99,(YEAR($A$26:$A$99)=YEAR(C$1))*(MONTH($A$26:$A$99)=MONTH(C$1))*($B$26:$B$99=$A2),"N/A")
Enter it in cell C1. Change the references to row 99 to the last row containing data. Change the text to whatever you want when no match is found. Copy the updated version down to row 23 and as far to the right as your table goes.
The formula works as long as there are not multiple matches, two entries for the same site in the same month.