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What is the error that you are receiving?
What does the error page say on it?
That error page is a '404' error. That basically means that the 'administrator' page doesn't exist. That could be due to a number of factors.
What version of Joomla are you using?
I was able to find the version, 3.9.4. The issue you are seeing is often caused by a bad .htaccess file. This file can be used to control access and settings for any given webpage, in this case, your administrator console. To troubleshoot, you can try renaming that file to something like .htaccess.txt and then see if the page loads. You want to do the one that is located in that administrator folder on your joomla installation.
Awesome. Please let me know if you need more help with this. Otherwise, please be sure to rate my answer 5 stars. Bonuses are always appreciated.
I don't see much about the htaccess file for the admin folder specifically, but joomla does provide one for the main site.