How do I obtain a license file to enable the design security features

Do you have a subscription license? If yes, check whether you have the following line in the license file:

FEATURE quartus_nonvolatile_encryption

(as mentioned in license FAQ: https://www.altera.com/support/support-resources/download/licensing/q-and-a.html#c_07)

If this line is missing, regenerate the license file in licensing center.

If you're using web edition, i think you might need to contact your local distributor and ask them to request a license from altera. Hope this helps!