If you get the follow error, you can try this solution:

$ sudo mv /var/lib/dnf/history /var/tmp/.
(from here) https://bugzilla.redhat.com/show_bug.cgi?id=1596540#c52

Then rerun dnf.

Work for me.

Thanks

Traceback (most recent call last):
  File "/usr/bin/dnf", line 58, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 179, in user_main
    errcode = main(args)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 64, in main
    return _main(base, args, cli_class, option_parser_class)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 99, in _main
    return cli_run(cli, base)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 123, in cli_run
    ret = resolving(cli, base)
  File "/usr/lib/python3.7/site-packages/dnf/cli/main.py", line 146, in resolving
    base.resolve(cli.demands.allow_erasing)
  File "/usr/lib/python3.7/site-packages/dnf/base.py", line 759, in resolve
    self._transaction = self._goal2transaction(goal)
  File "/usr/lib/python3.7/site-packages/dnf/base.py", line 656, in _goal2transaction
    ts.add_install(pkg, obs, reason)
  File "/usr/lib/python3.7/site-packages/dnf/db/group.py", line 256, in add_install
    ti_new = self.new(new, libdnf.transaction.TransactionItemAction_INSTALL, reason)
  File "/usr/lib/python3.7/site-packages/dnf/db/group.py", line 219, in new
    rpm_item = self._pkg_to_swdb_rpm_item(pkg)
  File "/usr/lib/python3.7/site-packages/dnf/db/group.py", line 210, in _pkg_to_swdb_rpm_item
    rpm_item = self.history.swdb.createRPMItem()
  File "/usr/lib/python3.7/site-packages/dnf/db/history.py", line 291, in swdb
    self._swdb = libdnf.transaction.Swdb(self.dbpath)
  File "/usr/lib64/python3.7/site-packages/libdnf/transaction.py", line 729, in __init__
    this = _transaction.new_Swdb(*args)
RuntimeError: Exec failed: no such table: main.trans_cmdline



-- 
Dario Lesca
(inviato dal mio Linux Fedora 28 Workstation)