Backups in Cpanel get placed under your root folder which is above public_html and the first thing you see when you go into Cpanel File Manager.

That message you see is actually in error. You can extract the file on the server or you can download and extract it on your computer with the right software. To extract it on the server, click on the backup file name and then choose Extract File in the menu on the upper right. It will send it to a subfolder by the same name in the same folder you are in (root). Once it is extracted, you can copy over what you need to the right folder. In File Manager, you would use the folder icons to navigate into folders and click on the name of the final folder where you want to copy the file. If you are restoring all of public_html, it may be easier for us to do it for you since we have access to shell and can do it with one command.