In order to use FLkey with FL Studio, you need FL Studio 20.9.2 or above installed.
FLkey integrates with FL Studio using advanced scripts built into FL Studio. Version 20.9.2 comes with the script integrated into the software itself.
If you have an earlier version of FL Studio installed you will need to update to 20.9.2 to get the best out of your FLkey: Getting Started With FLkey // Updating FL Studio
If you do not have FL Studio yet, FLkey comes with a 6 month trial of FL Studio Producer Edition to get you started: Getting Started With FLkey // FL Studio Producer Edition - 6 month trial