To find Plesk version, login to server as user root SSH and run
plesk version
Example
root@root861:~# plesk version Product version: Plesk Onyx 17.9.8 Build date: 2018/11/16 16:00 OS version: Debian 9.6 Revision: 6af2ef23569cac03e01272676158a016ed0eea1a Architecture: 64-bit Wrapper version: 1.2 root@root861:~#