What format does VisitDate come in? If it does come in as YYYYMMDD, then what you are using looks right except for the single quote around the field name. I would either use double quotes or square brackets
Date(Date#(VisitDate, 'YYYYMMDD'), 'YYYYMM') as [VisitYear Month],
Update: I guess it doesn't matter, single quotes seems to work as well.