Hi,
Could you eloborate more how and when you got this error message.
Looks like permission error on linux operating system. try changing permission using chmod 755 or other.
Prestashop use Tools:unlink function similar to php unlink function.
PrestaShop Tutorials Videos [How to do Tasks]
https://www.prestashop.com/forums/topic/907438-prestashop-tutorials-videos-how-to-do-tasks/