While you are setting up and finishing installation your Roundcube in your Ubuntu and you accidentally shut it down, the installer will be disabled and you will encounter a message shown below.

To enable it, follow the steps below:
Step 1: Open your terminal and go to your roundcube folder. In this case, it is in /var/www/html
cd /var/www/html/roundcube

Type ls
to display the list of the files and folders inside roundcube folder. There should be a folder named config.

Step 2: Check the files inside the config folder.
cd config
ls

Step 3: Edit the file named config.inc.php.

sudo nano config.inc.php
It will open a php file.

Step 4: At the bottom, paste the command below.
$config['enable_installer'] = true;

Step 5: Save it and exit. Press Ctrl + S and then Ctrl + X.
Step 6: Refresh the browser. It will show the same output as shown below.

Step 7: continue your setup.