There are a number of features/services which are incompatible with the Apple Watch Series 3 (GPS + Cellular). If you wish to activate the Apple Watch with cellular service, you must disable them from the paired iPhone.
Selective Call Blocking by specific numbers from the Preferences page of Sprint.com
Short codes starting with * or #
Google Voice with your existing Sprint number
TTY (Teletypewriter) Service
If you have disabled all of these features and are still unable to activate your Apple Watch Series 3, please create a post in the iPhone forum and an agent will assist you.