Note the help text that shows above these two choices:

"Select when to send this acknowledgement email. This applies if this part uses a Payment part that is configured to send a summary acknowledgement email."

If your Event Registration Form is not configured to use a Payment Part to process payments (i.e. it's set to "Proceed directly to payment"), the choices under the "More Options..." button in the Acknowledgement Email do not apply and will not save.

In this case, the Acknowledgement Email associated with your Event Registration Form part will always be sent.