Hi,
I'm using Unity 2020.3.18f1 with Hub and I'm having the same problem. I have a PlasticSCMPlugin.exe in D:\Unity\Hub\Editor\2020.3.18f1\Editor\Data\Tools\VersionControl, but not in D:\PlasticSCM5\client. I have the latest version of PlasticSCM and the Unity Version Control package version 1.9.0.
All the best,
Oliver