Login, click on the left side on "Ad" below the phrase "Create". Now after a while you will see a notice to turn off the adblocker.
Facebook's advertising tools might not work as expected when an ad blocker is enabled in a web browser. Turn off the ad blocker or add this web page's URL as an exception so you can create ads without any problems. After you turn off the ad blocker, you'll need to refresh your screen.