AMAZON.LastName not able to recognise last name given in Prompt in AWS LEX V2

If you already have a list of last names that you expect, you can create a custom slot type then use "Expand values (default)" in the custom slot to add all of the last names you expect. This will be used as a training data for the custom slot type.

As for the built-in slot type, there is no AMAZON.STRING slot type but you can use AMAZON.AlphaNumeric and specify regular expression to limit the field to only strings