Hi davidcockey,
The old SectionTools plugin used to install with a different ID. If you have an old SectionTools installed, then you’ll need to delete the old one and re-install.
This is how you delete the old SectionTools plugin:
Close Rhino, then open: C:\Users[USER]\AppData\Roaming\McNeel\Rhinoceros\5.0\Plug-ins
You will probably see 2 SectionTools folders, delete both, then re-install SectionTools.