./start.sh: 12: exec: java: not found

This error message means that the Java runtime environment (JRE) is not installed on your system or it is not configured properly. To resolve this issue, you need to install Java on your system and set up the environment variables correctly.

Here are the steps to install Java on Ubuntu:

1. Open the terminal by pressing Ctrl+Alt+T.
2. Update the package list by running the following command:

sudo apt-get update

3. Install Java by running the following command:

sudo apt-get install default-jre

4. Verify the installation by running the following command:

java -version

This command will display the version of Java installed on your system.

If you still face the issue, you may need to set up the environment variables. Here are the steps to set up the environment variables:

1. Open the terminal by pressing Ctrl+Alt+T.
2. Edit the environment variables file by running the following command:

sudo nano /etc/environment

3. Add the following line at the end of the file:

JAVA_HOME="/usr/lib/jvm/default-java"

4. Save and close the file by pressing Ctrl+X, then Y, and finally Enter.
5. Reload the environment variables by running the following command:

source /etc/environment

Now, try running the start.sh script again. It should work without any errors.