Function command issues

Hi, i have a file created in the functions file in a folder and re-open the client and join the world and type the /function foldername:filename but just comes up unkown function also has the # in it, not sure if it requires something else that its missing or its bugged also tried it in 1.12 Pre-Release 5 and still comes up unknow function also have the file as filename.mcfunction any ideas why its doing this