This is an issue generally noticed on the Pega Marketing 7.13 exercise system (which is used in the Pega Marketing Essentials 7.13 and other 7.13 based courses). When you have imported multiple JARs from previous exercises, sometimes it can corrupt the system and cause such issues.
To complete the exercises, the best approach we recommend is to keep a snapshot of the pristine image so that you can always return to it if something goes wrong. To take a snapshot of the pristine image, follows these steps after you have downloaded and set-up the exercise system, but before importing the first exercise JAR:
Start-up the VM.
Wait for the Pega log-in screen to appear in the browser.
Do a clean shutdown (just type shutdown in the VM window)
Then take a snapshot.
This is your pristine image snapshot.
Also, at the end of each exercise, take a snapshot, and start afresh from the pristine snapshot for the next exercise. Here's an example of how the snapshots appear in VirtualBox.