Hi all. I am late to the party. I am having the same issue.
I am using Unity 2019.3.0f6. I get an error message when I try to change the Version Control options.
I am not using Unity HUB.
I am not using alternative directories for Unity or Plastic SCM. Plastic SCM command line help comes up from the command prompt.
I just installed Plastic SCM, but I do not have the Plastic SCM plugin in C:\Program Files\PlasticSCM5\client\
The PlasticSCM plugin is installed in the Unity directory.
Any help would be greatly appreciated.
Thank you!
Fig