the structure is something like
if your 1st condition then
elseif your 2nd condition then
Hope this helps
Can you paste your full formula. If your second condition is to check null value you should go like this:
IF ATTR([iPMS-ReadinessAll].[Grant Agreement Type])='framework agreement'
DATEADD('month',4,ATTR([Final Merge (Disbursement Schedule and Reporting Dues 123)].[IPR2 due (Copie final)]))
ELSEIF ISNULL(ATTR([Final Merge (Disbursement Schedule and Reporting Dues 123)].[IPR2 due.....
Last else is optional.