We have checked using your ftp and found .htaccess file is missing in root of your site. refer this:https://prnt.sc/tgnv2f
Please contact your hosting provider for missing .htaccess file.
We are using "Authorization" key in headers to verify the user refer this :'https://prnt.sc/tgnub7
As in .htaccess file we add these lines to solve the issue:
SetEnvIf Authorization .+ HTTP_AUTHORIZATION=$0