Taxcode Not manually select during creation of po

Dear  Team,

Manually tax code Not picking  During PO creation.

I have search SCN but it  did not help.

My po's before accepting any tax-code according to user's requirement. But now did  not taking  any tax code according to user's choice

and even save without tax -code.

i defaulted a tax-code through me11 .Then it defaulted and picked.

but i  do  not  want  that  . I want manually select according to the list in the invoice tab.

Please Advise.

Ritesh