I have a problem upgrading to WordPress 3.6

have you tried:

-deactivating ALL plugins temporarily to see if this resolves the problem. If this works, re-activate them individually (one-by-one) to find the problematic plugin(s). If you can’t get into your admin dashboard, try deactivating via FTP or SFTP or whatever file management application your host provides. If applicible, also remember to deactivate any plugins in the mu-plugins folder. The easiest way is to rename that folder to mu-plugins-old.
– To rule out any theme-specific issue, try switching to the unedited default Twenty Twelve theme for a moment using the WP dashboard. If you don’t have access to your admin area, you can switch to the default theme by renaming your current theme’s folder in wp-content/themes and adding “-old” to the end of the folder name using via FTP or SFTP or whatever file management application your host provides. Alternately, you can remove other themes except the default theme. That will force your site to use it.
– resetting the plugins folder by FTP or PhpMyAdmin. Sometimes, an apparently inactive plugin can still cause problems (because the hooks remain unless plugins completely removed or some plugins stick around in cached files. So by renaming the folder, you break them and force them inactive). Read “How to deactivate all plugins when you can’t log in to wp-admin” if you need help.
– If the above troubleshooting steps fail to resolve the issue, try manually re-uploading all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory from a fresh download of WordPress. Make sure that you delete the old copies of files & folder before uploading the new ones. Read the Manual Update directions first!

[ Moderator note: personal attacked deleted ]

Inside wp-includes/post.php (so, the installation core file) there is a call to function (supposed to be defined inside the very same file) but the function definition is missing and it is gone at all !

This is ridiculous!

@ziggizag I’m sure Tara can read quite well. As you’re the one with a problem and need help, you may want to tone down that attitude.

These are 100% staffed by volunteer forums and Tara’s not the one with a broken installation.

Now before you react badly (please don’t, Tara and I are trying to help you) how far did you get with that advice? Particularly the parts about

– deactivating ALL plugins temporarily

…

– To rule out any theme-specific issue, try switching to the unedited default Twenty Twelve theme

…

– try manually re-uploading all files and folders EXCEPT the wp-config.php file and the /wp-content/ directory

Each one of those are part of the basic troubleshooting steps which hopefully will take care of this:

Inside wp-includes/post.php (so, the installation core file) there is a call to function (supposed to be defined inside the very same file) but the function definition is missing and it is gone at all !

The plugins and theme can be switched off using FTP or whatever file management tools your host has provided you with and don’t need a working WordPress dashboard for that part.