To start the Jenkins server please follow the below steps:
First, you need to start the Jenkins service from the terminal. Open the terminal in the VM and then run the below command:
sudo service jenkins start
Now open the browser in the VM(you can open Mozilla Firefox) and in the address path give the below path:
localhost:8080
It will open the Jenkins login page and the username and password will be already filled(no need to make any change). You can click on login and start working with Jenkins.
I hope this resolves your issue.