A 503 error indicates service is unavailable and could be for various reasons. Would be helpful if you provide a bit more details from the logs.
However you can check the following
Go to the Azure Portal: Logic Apps > Overview and check the status of the Logic App if it is enabled.
Restart the logic appYou can use diagnostics as well as indicated here:
(picture cited from https://learn.microsoft.com/en-us/answers/questions/2128372/getting-503-error-message-in-azure-apim-api-connec)
You can mark it 'Accept Answer' and 'Upvote' if this helped you
Regards,
Abiola