Nextcloud Impersonation Option is not working

NextCloud version: 15.0.2
Apache or nginx version (eg, Apache 2.4.25) : Apache
PHP version:


I am facing strange issue with Impersonation option on Nextcloud. I have removed all options (by default for Admins) from below textbox. I am not able to add any group in this text box and impersonation option is not working on my NC instance.