Dreamweaver local root folder does not exist

Going round in circles on Dreamweaver 21.3.  Mac OS 11.6.8.

Error on opening DW: "Local root folder does not exist".  I then go to Manage Sites, select my website and edit.  It is already pointing at the correct folder, but I have tried "browse for folder" to select the folder again, with no luck: I can "Save" and then when I click "Done" I get the same error message again.  It gives me the option to "Manage Sites" again, but when I click this button, the box disappears.  But nothing happens: I still cannot see or open any of my website pages for editing ("page not found").

My site is on an external hard disk, where I have permission to read and write.  File vault is off.  I've always used Dreamweaver to edit and upload my website.  I've tried using the other versions of Dreamweaver available (20.2.1 and 19.2.1) with the same issue.