You cannot use / in file names. This is a fundamental limitation of all file systems on all operating systems.

Note that Finder
appears to
(but doesn't actually) allow / in the file name because it translates / to : when you rename a file in Finder and then displays : as / for your convenience. You can therefore use : in your format instead of / to name your files. Sane applications like FileBot will display : as : but Finder will display : as / for historical reasons. I hope I'm being clear.

Personally, I'd avoid both /
(not possible) and :
(fake display value) in file names for the sake of sanity.

Please read this post for details:
https://stackoverflow.com/a/30816505/1514467