Older versions of Sigil on MacOS required that a newer system version of TclTk be installed before using Sigil plugins that use Tkinter. That is no longer the case with the latest versions of Sigil. Starting with Sigil 1.9.0, I believe.
It's mentioned in the Sigil Release Notes of all the versions wherever it's still necessary. A working TclTk .dmg is attached to the assets of the
Sigil 1.8.0 on Github, but I recommend updating to the latest Sigil if possible.