Q: SHARE BUTTON NOT WORKING

- Try resetting the SMC and NVRAM/PRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

https://support.apple.com/en-us/HT201295

About NVRAM and PRAM 

https://support.apple.com/en-us/HT204063

- Try starting in Safe Mode

OS X: What is Safe Boot, Safe Mode?

https://support.apple.com/en-us/HT201262

- Boot to Recovery and repair the startup disk

OS X: About OS X Recovery - Apple Support

https://support.apple.com/en-us/HT201314

- If it is repairable reinstall the OSX

How to reinstall OS X on your Mac - Apple Support

https://support.apple.com/en-us/HT204904

- If you do not have a backup use disk utility to restore the internal disk to an external disk so you can try to recover data.

Then format the startup disk and do a fresh install of OSX

This is the Mac Pro desktop forum. I requested your post be moved to the appropriate forum.

There's a problem with your OS X installation. The simplest way, but not the fastest way, to fix it is to reinstall the OS. Unless you're in a hurry or have metered network access, that's what I suggest, especially since I don't know whether there are other problems with the installation. Please back up all data before reinstalling. You won't need the backup unless something goes wrong.

The advanced shortcut below, if it works, may save you some downtime (and the cost of a multi-gigabyte download, if applicable.)

1. Back up all data.

2. Disconnect all external storage devices.

3. Start up in Recovery mode. Select a language, if prompted. The OS X Utilities screen will appear. One of the options is to install OS X. That's not what you're going to do.

4. This step is only necessary if you use FileVault 2. If you don't know what FileVault is, you're not using it. Go to the next step. Otherwise, launch Disk Utility, then select the icon of the FileVault volume ("Macintosh HD," unless you gave it a different name.) It will be nested below another drive icon. Select Unlock from the File menu and enter your login password when prompted. Then quit Disk Utility to be returned to the main screen.

5. Select Get Help Online. Safari will launch. While in Recovery, you'll have no access to your bookmarks, but you won't need them. Load this web page.

6. Triple-click anywhere in the line below to select it:

chflags -R norestricted /V*/*/S*/*/Caches

Copy the selected text to the Clipboard by pressing the key combination command-C.

7. Quit Safari. You'll be returned to the OS X Utilities screen.

From the menu bar, select

          Utilities ▹ Terminal

The Terminal application will launch. Paste into the Terminal window by pressing the key combination command-V.

Wait for a new line ending in a hash sign (#) to appear. Quit Terminal to be returned to the main screen.

8. Select

           ▹ Restart

from the menu bar.

9. This step must be carried out after the restart and while you're logged in as an administrator—not in Recovery mode. If you have only one user account, you are the administrator.

Select and copy this line as in Step 6:

sudo /usr/libexec/xpchelper --rebuild-cache

Paste into a Terminal window as before. You'll be prompted for your login password. Nothing will be displayed when you type it. Type carefully and then press return. If you don’t have a login password, you’ll need to set one before you can run the command. You may get a one-time warning to be careful. Confirm. You don't need to post the warning.

If you see a message that your username "is not in the sudoers file," then you're not logged in as an administrator. Log in as one and start this step over. You don't have to repeat the earlier steps.

Wait for a new line ending in a dollar sign ($) to appear below what you entered.

10. Do as in Step 9 with this line:

sudo /usr/libexec/*/app_sandbox_cache_builder --rebuild-cache

You can then quit Terminal.

Sharing options and Markup are missing after you install OS X Yosemite       also El Capitan