os.Mkdir is used to create a single directory. To create a folder path, instead try using:
os.MkdirAll(folderPath, os.ModePerm)
Go documentation
func MkdirAll(path string, perm FileMode) error
MkdirAll creates a directory named path, along with any necessary parents, and returns nil, or else returns an error. The permission bits perm are used for all directories that MkdirAll creates. If path is already a directory, MkdirAll does nothing and returns nil.
Edit:
Updated to correctly use os.ModePerm instead.
For concatenation of file paths, use package path/filepath as described in @Chris’ answer.