Hello Dor,
Welcome to the Microsoft Q&A and thank you for posting your questions here.
I understand that your deployment was successful because the pipeline completed, but the app might not have been deployed to the correct directory or was missing essential build files.
Ensure that the App Service resource exists in the specified resource group https://learn.microsoft.com/en-us/azure/app-service/overview also, identify errors during the startup phase of the Next.js application - https://learn.microsoft.com/en-us/azure/app-service/troubleshoot-diagnostic-logs and https://blog.xmi.fr/posts/terraform-app-service-slots
Another thing is your environment variables, connection strings, and settings should be corrected in Azure App Service - https://learn.microsoft.com/en-us/azure/app-service/configure-common
You can check if the app is correctly mapped to a custom domain in Azure App Service - https://learn.microsoft.com/en-us/azure/app-service/app-service-web-tutorial-custom-domain then, restart the application to resolve potential transient issues - https://learn.microsoft.com/en-us/azure/app-service/manage-apps
If the problem persists, use diagnostic tools or diagnose and solve problems in Azure App Service using this link - https://learn.microsoft.com/en-us/azure/app-service/overview-diagnostics or contact Microsoft support.
I hope this is helpful! Do not hesitate to let me know if you have any other questions or clarifications.
Please don't forget to close up the thread here by upvoting and accept it as an answer if it is helpful.