To restore the options to search your library and Apple Music alongside the iTunes Store, enable the iTunes Store in the Music app settings and verify your account connection.
Open the Music app on your Mac.
Click Music in the menu bar at the top-left of your screen, then select Settings (or Preferences).
Go to the General tab.
Look for the Show section and make sure a checkmark is placed next to iTunes Store.
Click OK, then completely quit and relaunch the Music app.
Click into the search field at the top of the sidebar; clickable context buttons or tabs for Your Library, Apple Music, and iTunes Store should now reappear on the right side of the search field or top right of the main window