Basically there are two ways to resolve the issue:
1. You can assign a new port to the app
2. You can stop whatever process is using the port
You'll find details on both approaches here https://community.mendix.com/link/space/studio-pro/questions/124068
Hi, Oluwadamilola Adewole
Please restart your pc and run the application or go to task manager close current running Mendix and browser also, reopen application and start again. it will work
I hope it will help you !
Thank you!