This is happening because you are entering an invalid URL for your site. To access your site you need to enter your URL exactly. Here's a few things to check:

the URL should begin with https:// not http://
the URL should not include www. For example, if your site is you should not enter
Only .com will work or any other suffix will not work

If you have forgotten your URL please take a look at the article below:

I've forgotten my login URL
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