If you are getting below exception for your .Net Core 3.1 Lambda function after deploying it via web page, continue to reading reason.
Could not find the specified handler assembly with the file name ‘LambdaTest, Culture=neutral, PublicKeyToken=null’. The assembly should be located in the root of your uploaded .zip file.: LambdaException
When you create lambda function via aws web ui, it set default handler as LambdaTest::LambdaTest.LambdaHandler::handleRequest. You need to change it as your your code.