The script worked when I tested in GNOME SHELL 3.36.6

[admin@ADMIN ~]$ gnome-shell --version

GNOME Shell 3.36.6

[admin@ADMIN ~]$ 

Here is the modified content under my $HOME/.profile file.

# start my custom script for setting random background wallpapers

if [ -f "$HOME/wp.sh" ] ; then

    bash $HOME/wp.sh &

fi

Here is the modified content under my $HOME/wp.sh file.

#!/bin/bash

# script to set random background wallpapers on my GNOME desktop

# set base path

export wallpaper_path=/home/admin/Pictures

shopt -s nullglob

# store all the image file names in wallpapers array

wallpapers=(

    "$wallpaper_path"/*.jpg

    "$wallpaper_path"/*.jpeg

    "$wallpaper_path"/*.png

    "$wallpaper_path"/*.bmp

    "$wallpaper_path"/*.svg

)

# get array size

wallpapers_size=${#wallpapers[*]}



# set wallpapers in incremental order

index=0

while [ $index -lt $wallpapers_size ]

do

    gsettings set org.gnome.desktop.background picture-uri "${wallpapers[$index]}"

    # index is maxing out, so reset it

    if [ $(($index+1)) -eq $wallpapers_size ]

    then

        index=0

    else

        index=$(($index + 1))

    fi

    # keep the wallpaper for the specified time

    sleep 1m

done

Wallpapers changed at 1m interval as expected.