I’ve tried to transfer data from old iPad to new one and it won’t complete transfer, what is wrong?

These support pages should get you started with setup of your new iPad - and the methods of transferring your data from old to new iPad:

Set up your iPhone, iPad or iPod touch – Apple Support

Transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

Use Quick Start to transfer data to a new iPhone, iPad or iPod touch – Apple Support

If the donor iPad, from which you are transferring your data precedes iOS12.4, QuickStart will not be available.

“Transfer” is rather a misnomer. In reality, your data and setup is merely cloned to the new device. Nothing is moved or removed from the original device.

An alternative is to restore either an iCloud or iTunes backup to the new device:

How to back up your iPhone, iPad and iPod touch – Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

If QuickStart is not available, or fails, try restoring an iCloud or iTunes backup of the old device to new.

If restoring from a computer, you’ll need access to either a Windows PC (with iTunes for Windows installed) or a Mac (with iTunes or Finder - as appropriate for the installed version of macOS).

Download and use iTunes for Windows – Apple Support

Update to the latest version of iTunes – Apple Support

If you don’t have access to a suitable computer of your own, that of a helpful friend or family member will suffice. Alternatively, the technicians at the Genius Bar of your local Apple Store or Apple Authorised Service Provider (AASP) will be delighted to assist.

Genius Bar Reservation and Apple Support Options - Apple

How did you attempt to do it? Through a Mac or PC? iCloud? Quick Start? The top article suggests that you tried using Quick Start, which I've tried before and found to be prone to stalls where it would never complete.

My recommendation has always been to use a computer and connection cables to do a transfer. It's the most reliable way.