Try using the Add/Remove Programs tool from Windows to remove any existing version of BandLab Assistant or Cakewalk by BandLab that appears. You can keep any older versions of SONAR if you have them, just remember to choose Do Not Remove Shared Folders if uninstalling any versions.
After, you can follow the steps from our How To: Download & Install Cakewalk by BandLab?
If the installation still does not proceed, please try re-installing Cakewalk by BandLab by running as administrator. To do so, please right-click BandLab Assistant and Run as Administrator.