Thank you for contacting Telerik Support.
From the provided code snippet only it would be impossible for us to determine what is causing the error that you are receiving. For your convenience I am attaching a page with a very simple example, where I am using the code-snippet that you have provided. As you could noticed, the PivotGridCalculatedItem is correctly added, so please see what exactly differs in your actual project.
If you continue to experience the same issue and in order for us to be able to assist you any further, please try to prepare a very simple, runnable project, replicating the same error, so we could inspect it locally and hopefully, isolate the root of the problem.
I am looking forward to your reply.
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