When used from an open checkout, fossil scrub --private yields the following error:

SQLITE_ERROR: no such table: main.delta
no such table: main.delta
CREATE INDEX IF NOT EXISTS delta_i1 ON delta(srcid);

CREATE TABLE IF NOT EXISTS shun(uuid UNIQUE);

CREATE TABLE IF NOT EXISTS private(rid INTEGER PRIMARY KEY);

CREATE TABLE IF NOT EXISTS reportfmt(
   rn integer primary key,
   owner text,
   title text,
   cols text,
   sqlcode text
);

CREATE TABLE IF NOT EXISTS concealed(
  hash TEXT PRIMARY KEY,
  content TEXT
);

It does work when specifying the repository on the command line with the -R option though, so it's not critical.