M-x spacemacs/toggle-fill-column-indicator
This command can turn on the fill-column indicator in Spacemacs, but how to turn on it at start up in Spacemacs?
Dunno what that Spacemacs command is, but if you use Emacs 27 or later you have minor mode display-fill-column-indicator-mode. You can turn that on in your init file using (display-fill-column-indicator-mode).
If you have an older Emacs release you can use library fill-column-indicator.el from Emacs Wiki. With that, put this in your init file: (fci-mode).
If your Spacemacs command is a minor-mode function, then you should be able to, similarly, just put its name between parens in your init file, to turn the mode on.