Hi everyone. I searched all over for an answer to this problem I too encountered and found an answer that worked here:
The short is, the folder and contents can't be compressed with windows. To find out open the properties menu with Right click > Properties and find the option on either the Properties > General tab or by clicking Advanced button found at the bottom of the General tab.
Make sure these are unchecked to successfully delete duplicates with ccleaner:
[ ] Compress this drive to save disk space (When viewing drive properties)
[ ] Compress contents to save disk space (When viewing folder or file properties)