Plex Media Server asks for permission to allow network connections each time I login.

I am running El Capitan on an iMac and I have set up my Firewall to allow network connections to Plex Media Server, but each time I sign in I get a popup box asking for Plex Media Server.app to allow network connections. How do I correct that?
I tried using the Terminal command: sudo code sign —force —sign - /Applications/Plex\ Media\ Server.app/ and I got the message: /Applications/Plex Media Server.app/: replacing existing signature
xcrun: error: invalid active developer path (/Library/Developer/CommandLineTools), missing xcrun at: /Library/Developer/CommandLineTools/usr/bin/xcrun
/Applications/Plex Media Server.app/: the codesign_allocate helper tool cannot be found or used
What do I do now?