The most likely cause of this issue is that no profiles have been assigned to the specific user account or any of the groups the user account is a member of.
To resolve this, login into the Administration Portal, navigate to Profiles Configuration and ensure that at least one profile is assigned to the user or group that is attempting to connect through the Desktop Client.
Access settings can be edited by editing an individual profile and selecting the Access tab, located on the far right as shown below.
If the user was a guest, ensure at least one profile is assigned to the user group designated for guests. See 'How Do I Enable or Disable Guest Access?' for further information on assigning a specific group guest access.