Fatal Error, Undefined Function….again.

I’m attempting to install the newest version of WP. This happens to me EVERY SINGLE TIME. And then I wonder why I don’t want to use WP anymore. I get this line when updating:

Fatal error: Call to undefined function: is_multisite() in /homepages/43/d283890383/htdocs/Blog/wp-includes/wp-db.php on line 505

I’ve tried the manual upgrate which seems to do nothing.

Someone help me please? This is the third time this stupid thing has happened to me, and last time I fixed it by deleting my plugins folder and then installing them again after-ward, but this time no luck!!