SeriesItems Collection in RadHtmlChart has been introduced in RadHtmlChart since Q2 2013 (more information about it is available in this
blog post from Stamo Gochev). It may be possible that your current project is not properly updated with the latest Telerik version and that is why this SeriesItems collection is not available yet.
What I can suggest you is to check whether the references of RadControls' .DLL files in your project and their version. It may be possible that the file versions in the GAC folder are the latest one but the project is referencing .DLLs from the BIN which are an older version. You can find a short video in the attached archive that illustrates how to check the .DLLs version and references for the current project.
If that is the case I can suggest that you try to upgrade your version of RadControls following the six easy steps from this
blog post by Marin Bratanov.
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