@FieldName not working

Trying to create a virtual field that returns a count of child records. I keeping getting "Method 'RieckApps.Rules.SharedBusinessRules.Combokey' not found."

I thought these context type fields were autogenerated as part of the app.

Note: This is the basic SQL I am using. SELECT COUNT(*) FROM ExemptCerts WHERE CertMgrkey = @Combokey
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