I have followed this instruction to install Brat and configure it:

sudo apt update
sudo apt install python3 python3-pip apache2
tar -xvzf brat-1.3_Crunchy_Frog.tar.gz
sudo mv brat-1.3_Crunchy_Frog /var/www/brat

Create an Apache configuration file for brat:

sudo nano /etc/apache2/sites-available/brat.conf

Added the following configuration:

<VirtualHost *:80>
    ServerAdmin [email protected]
    DocumentRoot /var/www/brat

    ScriptAlias /cgi-bin/ /var/www/brat/cgi-bin/
    <Directory /var/www/brat/cgi-bin>
        AllowOverride None
        Options +ExecCGI -MultiViews +SymLinksIfOwnerMatch
        Require all granted
    </Directory>

    <Directory /var/www/brat>
        Options Indexes FollowSymLinks MultiViews
        AllowOverride All
        Order allow,deny
        allow from all
    </Directory>

    ErrorLog ${APACHE_LOG_DIR}/brat_error.log
    CustomLog ${APACHE_LOG_DIR}/brat_access.log combined
</VirtualHost>

sudo a2ensite brat.conf
sudo systemctl reload apache2

sudo chown -R www-data:www-data /var/www/brat
sudo chmod -R 755 /var/www/brat
sudo ufw allow 80/tcp

When I typed on Chrome http://localhost/brat/cgi-bin/admin.cgi it showed “404 File not found”

I found later the cgi-bin file is missing in var/www/brat folder.