My MAc "EL CAPITAN" 10.11.3 doesn't shutdown.

What audio devices and software apps do you have?

See Known issues below > "Read Me" link

from > http://avid.force.com/pkb/articles/en_US/Compatibility/Mac-OS-X-10-11-El-Capitan -support-for-Media-Composer-Pro-Tools-Si…

...

Avid Audio Interface Drivers

• Pro Tools | HDX and Pro Tools | HD Native cards are supported with Pro Tools 12.3 to 12.4, OS X 10.11 and HD Driver 12.3 to 12.4. Please read HD Driver 12.3 Notes.
• Supported non-HD audio devices with OS X 10.11 (El Capitan):
  • 003
  • 003 Rack
  • MBox 3 Pro
  • MBox 3
  • MBox 3 Mini
  • Pro Tools | Quartet
  • Pro Tools | Duet
  • Eleven Rack
  • Fast Track C400
  • Fast Track C600
  • Fast Track Duo
  • Fast Track Solo

• Known Issues:

  • Restart/shutdown is prevented with HD driver 12.2.1 (and earlier) installed. Read Me

  • No audio output with latest USB drivers (e.g. Eleven Rack, Fast Track C400 & C600, MBox Mini, MBox)
    This issue has been solved with latest drivers (AAH-145)
• The following audio interfaces will not be supported on OS X 10.11 (El Capitan). The last supported operating system is OS X 10.10 (Yosemite).
  • Fast Track Pro
  • Fast Track Ultra
  • Fast Track Ultra 8R
  • Fast Track MKII
  • Mobile Pre MKII

If this is a plug-in that is somehow conflicting with your mac's operations, you could try deleting it.

Note the exact message you get about that audio plug-in.

Try looking at /Library/Audio/Plug-Ins and see if it is there; temporarily move it somewhere else. Then see after restarting there are no more complaints.

(the first time you're bound to still have the problem, as the plug-in will have been loaded into memory; after you restart once it should cause no more problems).

Howdy Luis

The Read Me page full text

===

Solution:

This is caused by an incompatibility between El Capitan and the Avid CoreAudio.plugin file.

• The Pro Tools | HD 12.3 (or later) driver installer (included with the Pro Tools installer) will automatically remove this file if El Capitan is detected. Therefore, if you update to El Capitan after installing HD Driver 12.3 (or later), you will encounter this issue. However, if you update to El Capitan before installing HD Driver 12.3 (or later), you will not.
• You may also manually delete the Avid CoreAudio.plugin file, located here: /Library/Audio/Plug-Ins/HAL/

Note:

Removing the Avid CoreAudio.plugin file will remove CoreAudio functionality with Pro Tools HDX and HD Native hardware (which doesn't work in most cases anyway on OS X 10.10 and higher as of the 12.3 release). A driver which allows CoreAudio functionality with Pro Tools HD hardware on the newer OS X versions is currently in development.

Workaround:

If you find yourself in a failed shut down situation where the menu bar has disappeared (but the dock is still visible), you can try the following procedures to complete the shutdown/restart:

• Click an app in the dock that launches quickly (a web browser, TextEdit, etc). The app will launch and the menu bar will return to the top of the screen. Click the Apple menu, and choose Log Out. After a couple seconds the shut down procedure should continue.
• Open Activity Monitor and Force Quit the WindowServer process.

Note:

There have been some reports of this issue occurring in Mavericks and Yosemite as well. The same procedures should apply to those OS versions.