I want to return information per month.

Can I do something like this;

=IF(MONTH=APRIL,COUNTIF(B:B,"CUSTOMER ERROR"),)

Obviously MONTH=APRIL is not the proper language, but I am having trouble
using other date functions.

I want to return a number of how many times a customer caused the default in
an order per month.

This was reccomended earlier:
=SUMPRODUCT(--(S2:S999="CUSTOMER ERROR"),--(MONTH(B2:B999)=5))

Still didn't work. Maybe a dumb question, but what does the "--" mean?

Any help is appreciated.
Thanks so much
Scott