This will work:
clc
clear
close all
I = imread('peppers.png');
[height, width,Channels] = size(I)
blankImage = ones(round(height/5),width,Channels,'like',I);
combineImg = [I; 255*blankImage];
imshow(combineImg);
First you needed to round the height dimension, and then since the image is RGB it has 3 channels, so the 3rd output of 'size' is 3, hence you had a conflict when trying to concatenate.
Here is what I got as output. Notice that there is a white image at the bottom but it does not show up well here. Try it in Matlab and it will work.
EDIT: Here is what it look like with a black blankImage.To get it, change
combineImg = [I; 255*blankImage];
with
combineImg = [I;blankImage];
Output:
