Some third-party Moodle course format plugins can throw a bogus "No guests here!" error message on your course page, even if guest access is appropriately enabled for the course.

Your Moodle theme is not at fault for this error. The issue lies with the third-party course format plugin developer. The error message appears in all Moodle themes, including the default Boost theme, but is sometimes obscured by a sticky header navigation bar, particularly Boost's default navigation bar. If you remove the Boost navigation bar via the web inspector, you'll see the error printed behind it. 

The best solution is to contact the third-party developer of the course format plugin to report the issue.

While you wait for a solution from them, you can implement this quick fix, will remove the error:

Via FTP, cPanel, or SSH, find the following file in your Moodle root:

/course/format/*COURSEFORMAT*/lib.php

Comment out line 205, reading:

echo $e->getMessage();

So that it looks like this:

// echo $e->getMessage();

After sucessfully completing the above steps, the bogus error message will disappear. However, for the optimal solution we recommend contacting the developer of your third-party course format plugin to alert them to the bug in their code.