Look at the Tomcat Manager http://<ip>:<port>/manager and see if the /prweb resource is started. If not, start it up. If it cannot be started, I suggest to follow the OP's solution and re-extract a new copy of the PVS.
* for the login credentials to Tomcat Manager, try admin/admin or admin/password.
Have you tried using my solution above, i.e. going to Tomcat Manager (http://<ip>:<port>/manager) to see if the prweb application is running? If you're still unable to resolve, I suggest that you just restart from scratch again with a new .ova from Pega Academy.
I am not sure I understand what you mean. Please do reply to my questions in the previous post first, so I can understand what course you are working on, and the name of the exercise VM you have installed. If you are not using the Pega Academy exercise system, then you may need to contact GCS or whoever you have obtained the VM from.
What course PVS are you working on? Check the Tomcat log files and see if you can identify what is the reason why /prweb cannot be started. If you are unable to resolve it, I suggest you contact firstname.lastname@example.org with details of the problems and what steps you had taken so far.