If you wish not to have the validation on Last Name and only on the First name, what you can do is first toggle off the First/Last Name option from settings, and then add two Custom Fields and rename them accordingly as First Name and Last Name and then apply the validation only on First Name.
Refer to screenshot: https://app.screencast.com/9n9f16NJOEQM9
Refer to screenshot: https://app.screencast.com/qcZlH4v04Zpms