Erase computer authorisation

I can't erase authorisation on my Mac.  I had to install 4.5 because I have brought a new Kobo reader but cannot get it connected/authorised.  I have changed my email address and the computer authorisation is under old address.  I tried to erase authorisation but keep getting the message "unable to erase authorisation. Please try after some time."  Have been trying for 4 days now.  I cannot get my Kobo to authorise either and this is possibly because of the computer authorisation.  I am getting fed up and very frustrated,  it's not user friendly.  I have a brand new Kobo and can't load any library books.  Desperate for help.