This is warning is due to not having the 777 permissions to /cache folder and all files and folders within it.
To change permissions, you can use a FTP program like FileZilla or you can even use File Manager from your cPanel.
We would recommend you use FileZilla to do this task. Changing the permissions is very easy.
First, make sure that you have an FTP account. If you don't have one - create an FTP account from cPanel.
Now, login via FTP using FileZilla and navigate to public_html/wp-content/themes/themename/
Right click on the cache/ folder (in the root directory of the theme) and select Permissions from the context menu.. Type 777 in the "Numeric Value" field and click on Recurse into subdirectories. Choose Apply to all files and directories. Finally click on OK.