{"message":"Missing required key 'Item' in params","code":"MissingRequiredParameter" code example

Example: Missing required parameter in input: "Key"

Check wether you are Writen Captial "Key" or "key"?

Correct: s3.get_object(Bucket=bucket_name, Key=file_name)
Wrong: s3.get_object(Bucket=bucket_name, key=file_name)