I need to extend data for iPad

What data plan are you referring to? If you have an iPad with cellular, you need to contact your cellular provider about extending your plan with them.

If this is not what you are asking, then you need to try and clearly explain what you need or want.

Hello. You cannot add storage on the iPad I highly recommend you purchasing a new iPad or getting iCloud storage so you can save here is a article on what it is Upgrade to iCloud+ - Apple Support

if it’s for data plan etc here are some major carriers

https://www.telstra.com.au/contact-us

https://www.optus.com.au/support/contact-us

https://www.att.com/support/contact-us/

My apology, I forgot to add the link. Here you go. Choose your country or region - Official Apple Support

You cannot put more storage on the iPad itself. If you are trying to update your iCloud storage and you are receiving a message, then there could be something wrong with your payment method. You need to contact Apple Support as indicated. Use this link to find your location, then contact iTunes Support.