Can't open dynamo in Revit 2021

[Kzs_David]

Thank you for your answers, I have solved my problem, here is how I solved it :

1. I first uninstalled the plug-in py.revit I installed. At this time, the problem changed from the message A to the message B.
2. And I try the method provided by others below:
   If you cannot launch Dynamo in Revit 2021 with NullReference IsOptedIn error
   
   
3. Finally, I can open my Dynamo in revit2021! And I reinstall my py.revit, it works too!