I am trying to calculate the date difference of table [Date] when table [AI Status] says "Pending". I keep trying the obvious DATEDIFF function with an IF statement but I always get an error. It seems very straightforward and I don't know I am struggling

The error message says " Cannot mix aggregate and non aggregate comparisons or results in "IF" expressions.

Here is the formula:

IF [AI Status] = "Pending" THEN DATEDIFF('day', ATTR([DB].[Date]), TODAY()) END

the ATTR was automatically added but does nothing when I remove it. I also added a then statement before and still nothing.