I ran into the following problem with dnf today: 

$ sudo dnf group install development-tools
Warning: Group 'Development Tools' is already installed.
Error: No packages in any requested groups available to install.

However, the group was not installed as can be seen here:

$ rpm -q gcc
package gcc is not installed
$ sudo dnf group erase development-tools
Error: No packages to remove from groups.

Before, I had installed the group and then did a rollback to the transaction ID before the grup install (I do this a lot and never ran into problems before).

I tried a few things and found the "group upgrade" to work:

$ sudo dnf group upgrade development-tools
Resolving dependencies
--> Starting dependency resolution
---> Package patchutils.x86_64 0.3.3-4.fc20 will be installed
---> Package diffstat.x86_64 1.57-3.fc20 will be installed
---> Package cvs.x86_64 1.11.23-33.fc20 will be installed
---> Package systemtap.x86_64 2.4-2.fc20 will be installed
...

But I canceled this and re-run the same command, this time I got this:

$ sudo dnf group upgrade development-tools
Warning: Group 'Development Tools' is already installed.
Error: No packages in any requested groups available to install.

Next I tried marking the group as not installed or installed:

$ sudo dnf group mark remove development-tools
Error: No matching installed groups found.

$ sudo dnf group mark installed development-tools
Traceback (most recent call last):
  File "/bin/dnf", line 35, in <module>
    main.user_main(sys.argv[1:], exit_code=True)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 242, in user_main
    errcode = main(args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 75, in main
    return _main(base, args)
  File "/usr/lib/python2.7/site-packages/dnf/cli/main.py", line 124, in _main
    cli.run()
  File "/usr/lib/python2.7/site-packages/dnf/cli/cli.py", line 1378, in run
    return self.command.run(self.base.extcmds)
  File "/usr/lib/python2.7/site-packages/dnf/cli/commands/group.py", line 215, in run
    return self._mark_install(extcmds)
  File "/usr/lib/python2.7/site-packages/dnf/cli/commands/group.py", line 109, in _mark_install
    names = set(g.name for g in itertools.chain(g.mandatory_packages,
NameError: global name 'itertools' is not defined
[mimox ~]$ sudo dnf group mark remove development-tools
Error: No matching installed groups found.

Lastly I ran "sudo dnf clean all" and "sudo dnf makecache". After this the group install worked again:

$ sudo dnf group install development-tools
Resolving dependencies
--> Starting dependency resolution
---> Package patchutils.x86_64 0.3.3-4.fc20 will be installed
---> Package diffstat.x86_64 1.57-3.fc20 will be installed
...