Can't add blank folders for year/month PSE 20

PSE 2020 on MacBookPr0- having trouble adding blank folders in folder-tree view. I want to create folders by year and month within year. New folders create as Untitled Folder, but when I change the name to, for example, "2015", the folder disappears and I get error message "The operation could not be completed. The error may have occurred due to invalid characters read only location or existing folder having same name". Thanks. Jim Hagen