Handler error when connecting lambda function to RDS database

I am trying to follow the tutorial in https://docs.aws.amazon.com/lambda/latest/dg/services-rds-tutorial.html to connect a lambda function to my database. I did not create a database from the tutorial, as I am trying to connect to an existing database. Whenever I run the code

aws lambda invoke --function-name CreateTableAddRecordsAndRead output.txt

I get the following error:

{"errorMessage": "Handler 'handler' missing on module 'app'", "errorType": "Runtime.HandlerNotFound", "stackTrace": []}.

I don't have any error with creating the lambda function. Does anyone know how I can fix this?