I m facing cellular error.How can i fix it although all updates are completed

Read again -> Related Article -> If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart

Try to optimize the range of the mobile network, see this Apple article:

If you see No Service or Searching on your iPhone or iPad

If you see No Service or Searching on your iPhone or iPad - Apple Support

See also -> Manually update your carrier settings on your iPhone or iPad

Manually update your carrier settings on your iPhone or iPad - Apple Support

If they are On, turn Off Hotspot & VPN.