If the tools in below screenshot don't work install the gnome-terminal first

sudo apt install gnome-terminal

then change the /usr/share/remmina/external_tools to look like below

#!/bin/sh
 
####################
# Main Script
####################
#gnome-terminal -e $(dirname $0)/$1
 
# if [ -x "/usr/bin/x-terminal-emulator" ];
# then
#  TERMNAME="/usr/bin/x-terminal-emulator"
# else
#  TERMNAME="gnome-terminal"
# fi
 
TERMNAME="gnome-terminal"
$TERMNAME -e "$1" &
 
#if [ "$2" = "1" ]
#then
#  echo "Hit a key to continue…"
#  Pause
#fi

• Xubuntu 18.04
• Xubuntu 20.04

https://gitlab.com/Remmina/Remmina/-/issues/584