Hi Bill

Ok, the runtime error '438' when importing a previous created plan I can also reproduce here on three different Windows 11 Pro systems of different builds and different hardware, so this error seems inherent to Windows 11 and I'm afraid I doubt there will be an answer for that as Planner Import references an old Windows library that I think is either missing or unsupported in Windows 11.

The latest error you posted when trying to install Planner I can't recreate here, maybe the installer file is corrupt or your copy has a double extension ending xx.msi.exe, also, Windows 11 gets touchy about running installers from the desktop.

Provided the installer ends with the .msi extension and is run from the downloads folder, or any other location except the desktop, then it should be permitted by Windows 11.
You could try holding down the shift key while simultaneously mouse-right-click on the installer and select the "Run as Administrator" option to see if that gets past the Windows Security pop-up ..but Shhhhh, don't tell Bob

Even if that installer works on your 'other' Windows 11 system you still won't be able to import old plans, only create new ones.

I'll have a fresh look later in the week and see if anything obvious stands out that is prompting the '438' error and maybe Bob will have got back to you before then with a solution.

William.