Before you Issue order with no target you need to Set Unit Property >> Energy >> 75 (or greater), because I'm guessing your sentry doesn't have enough energy.
If you don't want it to use energy you can use Create Effect on Unit (From Unit) >> Execute Sentry - Guardian Shield (Create Persistent) on *Your sentry* from *Your sentry*
@FearsoftheRainbow: Go
Before you Issue order with no target you need to Set Unit Property >> Energy >> 75 (or greater), because I'm guessing your sentry doesn't have enough energy.
If you don't want it to use energy you can use Create Effect on Unit (From Unit) >> Execute Sentry - Guardian Shield (Create Persistent) on *Your sentry* from *Your sentry*