The page is not accessible after installation

I installed flynax and then I received this message when I opened the page.

Forbidden
You don't have permission to access this resource.

Additionally, a 403 Forbidden error was encountered while trying to use an ErrorDocument to handle the request.

In the .htaccess file, I modified the following line and then it was fine.
Row 88-89

Original
# Return 403 Forbidden error.
RewriteRule .* index.php [F]

Modified
# Return 403 Forbidden error.
#RewriteRule .* index.php [F]

Why is this possible, where can the error originally be?