I pad pro 12.9 first gen

If you haven't updated for a long time as your signature indicates may be the case, update your iPad using your home computer with iTunes (Windows) or Finder (Mac).

See this link for help --> Update your iPhone, iPad, or iPod touch with iTunes - Apple Support

Or --> Update your iPhone, iPad, or iPod touch with the Finder - Apple Support

All iPad Pro models can support iPadOS 15.4.1, the current version.