Dear Ankur,
I found this explanation from a colleague of mine in relation to this issue.
"For PO creation, it is not allowed to purchase cross the company code in standard. So even if you assign the other companies in the group where the purchaser is assigned, the error message "purchasing across company codes is not allowed" still persists. This is standard design."
In order to solve the issue, please use the "Select Company Code" from note 1079525
I hope this helps.
Kind regards,
Conor