Thank you for your post.
Pull down the File menu and select "Sharing -> Instant Web Publishing...". Try turning on Instant Web Publishing. If this turns on, then you should be fine. If not, try changing the port to 591 (registered with FileMaker, Inc.). If this still doesn't turn on, then there is something on your machine that is stopping Instant Web Publishing from turning on. Quit FileMaker Pro and reboot the computer. Once rebooted, do not launch any other application except FileMaker Pro and try turning on Instant Web Publishing. If successful, then you had a conflict with another application. If you still cannot turn on Instant Web Publishing, then perhaps Instant Web Publishing was not installed with FileMaker Pro. Uninstall and reinstall FileMaker Pro.
Any other information you can provide may be helpful in narrowing down possible causes.