Some customers have found that when performing the ArcGIS for Desktop offline authorization process, they are unable to save the authorize.txt. This applies to Single Use or Concurrent Use licences.
If you encounter this problem, in the first instance please check that you have local administrative privileges on the machine, and write access to the location where you are trying to save the authorize.txt file.
If this doesn't fix the issue then, unfortunately, the only resolution we are aware of is to uninstall and reinstall the software.
If you still have issues, please contact Esri UK Technical Support.