This article explains how to use a High Availability device in Standalone mode.
There are two scenarios:
1. The HA unit is an individual unit by itself, meaning it was not bought as a pair with the other unit. In this case, licenses have to be purchased to get services on this unit.
2. The HA unit is an actual High availability device. These devices are usually sold as a pair, reducing the overall cost of the hardware. To use this device in standalone mode, the "HA Conversion License to Standalone unit" license has to be purchased along with the service licenses on the firewall.