If y'all wish to look up the CreateRequest class you will find an attribute called "tag".

https://docs.microsoft.com/en-us/powerapps/developer/data-platform/org-service/use-messages#:~:text=tag,from%20the%20Organization%20Service

var account = new Entity("account");
account["name"] = "Contoso";

var request = new CreateRequest() { Target = account };
request["tag"] = "This is a value passed.";

var response = (CreateResponse)svc.Execute(request);

It works fine at depth 1 of a chain of plugins but, as soon as I try the same syntax at depth 2, I get the error message stated in the subject above.

Is it immutable?  How are we supposed to change the value of the SharedVariable?  Shouldn't this just reset the value or "tag"?

And don't even start with ParentContext.SharedVariables.  I don't know what MS has done to make sure that doesn't work (it does NOT contain or containkey!)

TIA

PS- I suddenly have another idea.  Change the "tag" SharedVariable instead of the request attribute -- I will report back.