What is the value that I have to enter for the FLEX parameter ? Also why does Taub change from iteration to iteration ?

The value you enter for the FLEX parameter is the value of Taub that is considered for the first iteration. Taub is a function of Pr/Py where Pr is the axial force in the member and Py the yield strength. Although Py (= AFy) cannot change but Pr changes between iterations because the axial force changes due to Pdelta effects (both large and small) and due to change in member stiffness. The Taub will not see the effect of the change in Pr until later exceeds 50% of the yield strength. Once Pr > 0.5 Py, Taub values are no longer 1 but are calculated using the equation provided for Taub in the AISC 360 code