Q: need Network Utility for El Capitan

You may be trying to run an outdated copy of the app on the Desktop or anywhere other than its original location in the System folder. If so, please delete the copy and use the one in the System folder. Never move or copy built-in applications.

If you get the alert when you launch the app by double-clicking its icon in the System folder, see below.

If you don't already have a current backup, please back up all data, then reinstall the OS.* You don't need to erase the startup volume, and you won't need the backup unless something goes wrong. If the system was upgraded from an older version of OS X, you may need the Apple ID and password you used.

There are ways to back up a computer that isn't fully functional. Ask if you need guidance.

If you installed the Java runtime distributed by Apple and still need it, you'll have to reinstall it. The same goes for Xcode. All other data will be preserved.

*The linked support article refers to OS X 10.11 ("El Capitan"), but the procedure is the same for OS X 10.7 ("Lion") and later.