first welcome to tableau and the forum
that error is generated when one of the fields in a formula is aggregated and others are not
you blended the data (without seeing the TWBX workbook I can't comment it that was the right thing to do)
so you are bringing a field from your secondary source to the primary - when you do that the field is aggregated - in this case with ATTR()
so in your formula [date] needs to be aggregated -- you can use min(), max() or attr() whichever makes sense - since you are comparing it to Today (a number) you should use min()
IF DATEDIFF('day',min( [Date]), DATE(TODAY())) <= 60 AND ATTR([Sheet1 (Source Data2)].[Record Update])="No"
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IF DATEDIFF('day', ATTR([Date]), ATTR(TODAY())) <= 60 AND ATTR([Sheet1 (Source Data2)].[Record Update])="No"