Hi Sam
Before Choosing the custom SQL in Edit query if you have any query filters which set to optional prompt then this error might come up.
It doesnt look like the error is because you are adding a code to the query , but because it already has some prompts which are optional and they seem to be not allowed in custom SQL .
If you have any optional prompts then make it a mandatory prompt and then check if you get the same issue.
Thanks,
Ashish