This formula works:

=SUMPRODUCT(--(Data!$L$7:$L$9998='Monthly
Summary'!A18),--(Data!$AE$7:$AE$9998='Monthly Summary'!$C$1))

I wanted to add one more condition and wrote the below based on that:

=SUMPRODUCT(--(Data!$L$7:$L$9998='Monthly
Summary'!A35),--(Data!AE7:AE9998='Monthly
Summary'!$C$1),--(Data!K7:K9999='Monthly Summary'!A13))

The working formula is great. Where did my sintax go wrong or what is
happening to give me a value error?