Rolling Average and Variance DAX Measures

You should be able to mark your table Date as a date table.  

This should clear up the error and allow you to move forward.  

If trying to mark it as a date table gives you errors along the lines of needing unique values, then you like need a date dimension.

Here's the power query code I use to create quick date dimensions. Be sure to update the Start and End dates as appropriate for your data.

let
    StartDate = #date(2019,1,1),
    EndDate = #date(2019,12,31),
    NumberOfDays = Duration.Days( EndDate - StartDate ),
    Dates = List.Dates(StartDate, NumberOfDays+1, #duration(1,0,0,0)),
    #"Converted to Table" = Table.FromList(Dates, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Renamed Columns" = Table.RenameColumns(#"Converted to Table",{{"Column1", "Date"}}),
    #"Changed Type" = Table.TransformColumnTypes(#"Renamed Columns",{{"Date", type date}}),
    #"Inserted Year" = Table.AddColumn(#"Changed Type", "Year", each Date.Year([Date]), type number),
    #"Inserted Month" = Table.AddColumn(#"Inserted Year", "Month", each Date.Month([Date]), type number),
    #"Inserted Month Name" = Table.AddColumn(#"Inserted Month", "Month Name", each Date.MonthName([Date]), type text),
    #"Inserted Quarter" = Table.AddColumn(#"Inserted Month Name", "Quarter", each Date.QuarterOfYear([Date]), type number),
    #"Inserted Week of Year" = Table.AddColumn(#"Inserted Quarter", "Week of Year", each Date.WeekOfYear([Date]), type number),
    #"Inserted Week of Month" = Table.AddColumn(#"Inserted Week of Year", "Week of Month", each Date.WeekOfMonth([Date]), type number),
    #"Inserted Day" = Table.AddColumn(#"Inserted Week of Month", "Day", each Date.Day([Date]), type number),
    #"Inserted Day of Week" = Table.AddColumn(#"Inserted Day", "Day of Week", each Date.DayOfWeek([Date]), type number),
    #"Inserted Day of Year" = Table.AddColumn(#"Inserted Day of Week", "Day of Year", each Date.DayOfYear([Date]), type number),
    #"Inserted Day Name" = Table.AddColumn(#"Inserted Day of Year", "Day Name", each Date.DayOfWeekName([Date]), type text)
in
    #"Inserted Day Name"