I bought CCleaner some time ago and have a fully working licence. However, when I recently upgraded my PC I wanted to reinstall CCleaner, but it would not download. The message I got was "Couldn't download". I have no problems with download anything else, so I wondered what was stopping CCleaner downloading???
This sounds like it may be a newly installed/updated Windows 10?
In Windows go to:
Settings>Apps>Apps & features and make sure that you don't have it set to allow apps from the 'Microsoft store only'.
If that isn't the problem then try downloading the 'Slim' installer from here (3rd one down):
https://www.piriform.com/ccleaner/builds
If it's still not downloading then please tell us which antivirus you are using? (And which OS you have).