According to Cisco.
NAT66—Translates IPv6 packets to a different IPv6 address. We recommend using static NAT. Although you can use dynamic NAT or PAT, IPv6 addresses are in such large supply, you do not have to use dynamic NAT.

Firepower Management Center Configuration Guide, Version 7.0

You have to start by defining the alert type

There are 3 types of NATs on a FTD:

1. NAT Rules Before. This is equivalent to Twice NAT (section 1) on classic ASA.
2. Auto NAT Rules. Section 2 on classic ASA
3. NAT Rules After. This is equivalent to Twice NAT (section 3) on classic ASA.

https://www.cisco.com/c/en/us/support/docs/security/firepower-management-center/212702-configure-and-verify-nat-on-ftd.html