Because the images you are trying to import are already in the LR Catalog, In the LR database file.
Yes, that seems to be the problem.
However, the already in the database of lightroom "existing" pictures are not more as files in the filesystem.
If I copy those pictures into the (old) or a new location and try to synchronize,
lightroom tells me that
want to (synchronize the already (in the database existing ) folder it tells teh same
"Es wurden keine zu importierenden Fotos oder Videos gefunden"
If I try to synchronize a folder containing already existing files of pictures as well as new ones,
it tells me exactly the same.
PS. All happens after having migrated all software to a new MAC (just a newer MacBook)
Any idea what to do?