OK. I may be onto something (at least I hope so!). In order to successfully install Python 3.8 on Windows 7. You need have SP1 for Windows 7 installed AND you need to have "Update for Windows 7 (KB2533623)" installed after that.
Since the Sigil installer checks for SP1 before installing, I'm beginning to suspect you don't have the KB2533623 update installed as well. Since we're not technically
installing Python with the Sigil installer, it wouldn't know if that update was installed or not.
The lack of that Python 3.8 pre-requisite is what I'm hoping is causing the errors with Sigil's bundled Python on your system.
Unfortunately that standalone KB2533623 update isn't available from Microsoft's update servers for WIndows 7 any more, but it has been incorporated into one their "Monthly Security Rollup" updates. Namely the KB4457144 update. Windows update should be able to install that. If not you can download the standalone package from the Microsoft update catalog:
https://www.catalog.update.microsoft...px?q=KB4457144. Make sure you download the one for 64-bit windows 7.
Might be a long shot, but it's all I've got at this time.
