The following error message is displayed trying to rename the system user at Plesk > Domain > example.com > Web Hosting Access:
PLESK_ERROR: Unable to execute usermng: usermng: /usr/sbin/usermod execution failed: usermod: user johndoe is currently used by process 31788 usermng: Unable to modify user: johndoe
Plesk bug #PPPM-10717 which is planned to be fixed in future product updates.
As a workaround:
2. Suspend the subscription under Domains > example.com > Suspend;
3. Disable “PHP Support” on Domains > example.com > Hosting Settings;
4. Stop “PHP-FPM 7.X” service under Tools & Settings > Service Management;
5. Rename the system user at Domains > example.com > Web Hosting Access;
6. Start “PHP-FPM 7.X” service again at Tools & Settings > Service Management.
Note: Replace PHP-FPM 7.X for the PHP version which the domain is using.