RT won't show folders/files on my desktop iMac Big Sur

I’m assuming for the moment that you are talking about the one that is shown here.

If this is so than you are using an old (February 4, 2020) version that should not be used any longer. This old version has bugs that are already fixed, lacks support for newer gear and does not have some of the new/upgraded functionality that is present in a more recent version.

Have a look here for an up-to-date version: