From what I can tell, Date Marker is being used to return a list of dates. When you enter your IF statement, you are checking if tbl[DATE] is greater than your list of dates. Since you haven't provided a method of aggregation of that list, Power BI is unsure what you are trying to do.
The points I would raise here are:
How is [Date Marker] any different in logic to simply using tblVisit[MaxDate] inside a formula? Is the date marker measure doing what you intended it to do?
What logic is your IF statement supposed to be? Are you trying to get a list of dates and you want tbl[Date] to be greater than all of the dates aggregated together?
Reading your various measures, they are written more like custom columns. I don't think this is what you intended.
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