Hi Faizan,
In Vendor master, you enter witholding tax information and that is not same as what you maintain in FTXP.. so it is ok that you cannot see in FTXP what you are maintaining in Vendor master record FK01.
The witholding tax codes are meant to deduct TDS and are created in configuration in SPRO in the node Financial Accounting>Financial accounting global settings>witholding tax>Extended witholding tax>Calculation> witholding tax code> Define witholding tax codes, whereas the tax codes in FTXP are meant for tax calculation on purchases and sales.
Check the account group of the Vendor(in FK03, Extras>Administrative data) you are trying to enter the witholding tax code.. it is probably the One time vendor and one time Vendor is not supposed to have repeat transactions so it is not allowing you to select a witholding tax type which has accumulation activated in its configuration.
Hope this helps..
BR,
Kavita