Hi Sweety,
Since the application is opening successfully on localhost, but the Debugger is returning 503 ServiceUnavailable, it looks more like an issue with the local debugger/runtime connection rather than the application itself.
I noticed you mentioned that the same application works correctly with Mendix 10.24.13. Since you're now using 11.12.0, I would first try updating Studio Pro to the latest 11.12.x patch. Mendix has released fixes in the 11.12.x patches, so testing on the latest patch would be a good first step.
You can also try:
Run Locally first and confirm the application is fully started before selecting Run → Debug. Kindly mark this as the accepted answer if it helps.