To be clear, check that your Avid MediaFiles folder (if you are using Avid-generated media) is at the top level of whichever drive you are using and the subfolders are correct: Drive>Avid MediaFiles>MXF>1 etc. The only exception to this is on recent clean installations on Mac Catalina and later when the Avid MediaFiles folder will be created within Shared Users on your boot drive (the boot drive isn’t the best place for your media). from the MC 2020.4 WhatsNew:
“With the release of macOS v10.15.x, you cannot write media to the root of the boot drive. Therefore, the default location for the Avid MediaFiles folder has changed with this release. You can now find the Avid MediaFiles folder in the following locations:
(Windows) drive:\Users\Public\Documents\Avid Media Composer
(macOS) /Users/Shared/AvidMediaComposer”