If you are getting the following error while creating the subdomain through the Plesk control panel then please refer to the following points to resolve the issue:

Error: skelmng failed: Access is denied. (Error code 5) at retrieves attributes for file or directory C:\Inetpub\vhosts\demo.com\test.demo.com
(Error code 1)
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Login to the windows server and open the command prompt:
Execute the following commands:

"%plesk_cli%\repair.exe" --repair-webspace-system-users -webspace-name demo.com
"%plesk_cli%\repair.exe" --repair-webspace-security -webspace-name demo.com
Replace demo.com with your domain name.

Hope this will help you to resolve the issue !