The error occurs on a small frame

[piratkin]

The error occurs when the multi-vterm-current-window-height is equal to the multi-vterm-selected-window-height.

 (defun multi-vterm-dedicated-get-window ()
   (setq multi-vterm-dedicated-window
         (split-window
          (selected-window)
          (- (multi-vterm-current-window-height) multi-vterm-dedicated-window-height))))

which is defined

(defcustom multi-vterm-dedicated-window-height 30 ... )

which is what causes the error

(error "Window #<window 3 on *scratch*> too small for spli...")

To solve the problem, I wrote:

(use-package multi-vterm
  :ensure t
  :config
  (progn
    ...
    (setq multi-vterm-dedicated-window-height (/ (frame-height) 2))))

P.S. I hope you find a more elegant solution