The ExtensionStoresup3 uses the older style WebDialogs for its dialogs - for backwards compatibility.
These dialogs need Internet Explorer to make them.
If [as you appear to say] you have 'uninstalled' Edge you've probably broken IE at the same time ?
Although you might not use IE for your day-to-day browsing it is vital for SketchUp's correct functioning...
Once you have IE set up you need to check your Internet Options [Control Panel].
This should be set to use Medium/Low for Local-Security and in Advanced styles/control etc and scripts etc should be allowed to run unhindered.
Because the dialogs are populated using javascript links between SketchUp and the dialog's HTML I suspect that the plugins-folders are not being listed in the dialog because the page's javascript is blocked... hence the 'security warning' ??