@Bry is right here. If that is SB2 from the above diagram, there is no neutral being sent from SB1.
2 possible solutions I see here. I’m going to put in another plug for an AUX switch. Then you can do a little rewiring in SB1 to use the AUX switch which would then send a neutral into SB2 and you can use it on both switches IF they’re controlled by the same circuit in your box (when you flip the breaker does it turn off both switches??)
A dimmer with no neutral could work as well depending on the type of lights in your garage. Most garages use fluorescent lamps, and not all of them are designed to be dimmed.