Thanks for asking question! The Service unavailable error means the web site's server is simply not available right now. This problem is often caused by Application level issues, such as:
• Requests taking a long time
• Application using high memory/CPU
• Application crashing due to an exception.
You can check your limit and usage in portal by going to "Diagnose and solve problems" section of your App Service blade, selecting "Best Practices", "Best Practices for Availability, Performance", and then "Temp File Usage On Workers".
Checks and verifies if the Temp File System usage on worker instance is nearing their quota limit. Temp files are located in 'D:\local\Temp' and 'D:\local\AppData' folder. This does not account for site's content.
Please note that the displayed usage and limits are per worker, and are aggregated across all apps in the same app service plan.
To add to this if you are running on Linuxfree SKU, Consider running a production application on a Standard, Premium, or Isolated App Service Plan for better performance and isolation. These SKUs are best for production workloads.
SKU options (Free, Shared, Basic) is recommended if you are still in testing mode.
In case issue persists please send an email to AzCommunity[at]Microsoft[dot]com referencing this thread.