While it is unfortunate that the UI is not providing a nice error message, this should always fail. Note the error of "Client xyz is logged in to abc - unable to delete until it's logged out". You cannot delete a client that is still logged into the target and therefore you must log out the client first. Therefore, the severity should be reduced and this should not be a test blocker unless you cannot delete a logged out client.
*** Bug 1786258 has been marked as a duplicate of this bug. ***
Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.
For information on the advisory, and where to find the updated
files, follow the link below.
If the solution does not work for you, open a new bug report.