Pivot table errors when refreshing

Hi There
I use the below formula in Column L to take date values from Column K to expedite a frequency (monthly, annually 3 yearly, etc) of next due dates in Column L - this in a table

=IF(K493<>"",EDATE(K493,LOOKUP([@Frequency],{"Annually";"Bi-Annually";"Every 10 Years";"Every 3 Years";"Every 5 Years";"Monthly";"Quarterly"},{12;6;120;36;60;1;3})),"")

I have created pivot tables with these tables and they were working fine all good with the above formula in place - Until today when I created a power query between three tables that has noting to do with the table in which this formula is

here is the message i got
Pivot Error.JPG


BESIDES THE FACT THAT THEY WERE WORKING FINE - could it possibly be that column K is blank, but it is not relay blank right? because it holds a formula - therefore the Pivot table picks up text in stead of date

I am making this statement because when i deleted the pivot tables and wanted to create new ones - I included the Due Data which is in Column L and I recieved the below error message

Pivot table.JPG