The lookup condition would not work because qlik wont be able to find the null entry for a particular customerid,as there is no entry of customerid for particular month.
For example: Customer Id 'A' has no entry in raw data for month 'Feb', 'Apr', 'Jun' hence it wont be able to find any null value for this customer id for these months so,your condition:" isnull(LOOKUP( ATTR( [Renewal_Contract] ),0 )) " will always be false.
So create the customer id entries for missing months in the raw data file.
A colleague of mine has given me a solution (using data transposition and next using case) , but it is impossible to maintain. A VlookUp function is really missing in this case. I'm not going to do my data preparation with Tableau and then use Qlik 😂.
I wonder if it is possible to optimize this code and just create a loop that does the job for all the months.