The on-screen instructions are clear - in that you must first restore your iPad before attempting to (re)activate your iPad:
This support page outlines the procedure to restore the iPad to Factory Settings:
Restore your iPhone, iPad or iPod to factory settings – Apple Support
To do so, 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
Use of a PC or Mac computer is unavoidable. 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