During initial setup of a new iPad, you should be provided opportunity to update the iPad to the most recent version (iPadOS 14.6) prior to commencing data transfer from old to new iPad - or attempting to restore from a saved device backup.
As you may be aware, data transfer or restoration of an iCloud/iTunes backup can only occur to a device with the same or later version of iOS/iPadOS.
You essentially have two choices…
1) You can restart the setup process (allowing the system update to complete prior to attempting transfer or restore), or
2) Complete the initial setup without transferring data - update the iPad to iPadOS 14.6 - and then either begin the transfer again - or restore data from an existing backup.
After the update, if you wish to begin the transfer process again, reset from iPad settings and begin the transfer process again:
Settings > General > Reset > Erase All Content & Settings
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
