When I try to access a document's root directory of a subdomain from Parallels Plesk Panel File Manager, the following error is displayed:
Error: Unable to change directory to /subdomains/<subdomain_name>/httpdocs: filemng failed: filemng: opendir failed: Permission denied
System error 13: Permission denied
Error: Unable to change directory to /web_users/<user>: filemng failed: filemng: opendir failed: Permission denied
Parallels Plesk Panel File Manager uses a system account configured to manage the current domain. If a separate system user is assigned to manage a subdomain, then it is not possible to manipulate its files using File Manager. It is not possible to use File Manager in order to access files of a webuser because its files always belong to a separate system account.
Choose the "Use the FTP user account of the main domain" option while setting up hosting preferences of a subdomain in order to be able to manage the subdomain using File Manager from within the main domain account.