That means you have not installed ActiveState's ActiveTCL package when you installed Sigil. It is required for tk based Sigil plugins to work.
See this part of every Sigil Release Announcement:
Quote:
Mac users should still download and install ActiveState's ActiveTcl Community Edition to utilize plugins that use Tk/Tcl GUIs. More here.
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and the link goes to
https://github.com/Sigil-Ebook/Sigil...Mac-OS-X-Notes