Unable to create subdomain via Plesk graphical interface: Please wait message is spinning continuously until Plesk session is expired.
This is Plesk bug with ID PPP-44951, which has been fixed in Plesk Obsidian 18.0.20 on 22 October 2019.
Consider installing latest Plesk updates: How to install Plesk updates
If update is not possible the following workaround is available:
- Log into Plesk;
- Enable website preview on a Plesk domain under Tools & Settings > Website Preview > Quick Preview on a domain name in Plesk: