Symptoms
An attempt to perform one of the following operations in Plesk might fail:
Creating a Subscription
Changing the password of the main Subscription system user in example.com > Web Hosting Access
Deploying an application (e.g., WordPress)
Settings Permanent SEO-safe 301 redirect from HTTP to HTTPS at Plesk > Domains > example.com > Hosting settings
The operations might fail with the following error:
Error: no secure shell available
Cause
The pre-defined shell set in the file /usr/local/psa/admin/conf/site_isolation_settings.ini overrides the settings configured at the Service Plan level:
# egrep "shell.*=" /usr/local/psa/admin/conf/site_isolation_settings.ini
shell = /bin/bash
Resolution
Note: The solution described in this article needs to be applied by the Plesk server administrator via SSH. If server access is not possible, please contact the service provider.
Connect to the server via SSH
Back up the configuration file
/usr/local/psa/admin/conf/site_isolation_settings.ini:# cp -a /usr/local/psa/admin/conf/site_isolation_settings.ini{,.backup}
Comment out the forced
shellparameter from the configuration file/usr/local/psa/admin/conf/site_isolation_settings.ini:# sed -i.bak 's/shell.*=/;shell =/g' /usr/local/psa/admin/conf/site_isolation_settings.ini
Control the SSH access permissions in the corresponding Service Plan or per-subscription:
Service Plan: Log in to Plesk GUI > Service Plans > Service Plan Name > Permissions > Management of access to the server over SSH > Can allow access only to a chrooted environment or Can allow access to any type of shell > Update & Sync
Subscription: Log in to Plesk GUI > Subscriptions > example.com > Customize (Right-sidebar) > Permissions > Management of access to the server over SSH > Can allow access only to a chrooted environment or Can allow access to any type of shell > Update & Lock

In case the issue persists, enable the Setup of potentially insecure web scripting options that override provider's policy option in the corresponding Service Plan or per-subscription:
Service Plan: Log in to Plesk GUI > Service Plans > Service Plan Name > Permissions > Enable Setup of potentially insecure web scripting options that override provider's policy option > Update & Sync
Subscription: Log in to Plesk GUI > Subscriptions > example.com > Customize (Right-sidebar) > Permissions > Enable Setup of potentially insecure web scripting options that override provider's policy option > Update & Lock