RESOLUTION
Full message in action log looks like:
"Process provisioning for order; Plesk client was not found for HSP account #1 on HN #1 or failed to create: Can not add IP address to Plesk client pool: Can’t add IP to IPpool, error: IP 192.168.1.1 does not exist or belongs to IP pool of another client"
This can happen in case if IP address by that Plesk node is registered in HSPcomplete, is marked as exclusive on Plesk. Change IP address type to shared in Plesk admin panel "Server > IP addresses management". This will solve the problem.