why i cant purchase app from ipad with my apple id ?

You don't actually remove the old ID - you sign out of that ID and sign into the new one.

Go to Settings>Store>Apple ID. Tap on the ID (assuming that the current one showing is the old ID) and then tap sign out. Then sign into the new ID.

Any purchases that you made with your old ID are always tied to that ID and must be updated by using the old ID.

If you are getting a message to contact iTunes support - that is what you should do. Follow whatever link or message that they give you and then see what happens.

It could be any number of reasons why this is happening, credit card issues, ID problem ... but iTunes will probably have to sort it out for you if you are getting that message.

Contact them and post back with any additional information.

This is the US link but I'm sure this is not what you want.

http://www.apple.com/support/itunes/contact/

Here is where you can link to support in other countires.

http://www.apple.com/support/country/