Final answer:
The correct Slate command to generate a new Slate project when running Yarn is 'yarn create slate-theme my-new-theme'.
Step-by-step explanation:
The correct Slate command to generate a new Slate project when running Yarn is yarn create slate-theme my-new-theme. This command creates a new Slate theme and names it 'my-new-theme'. It is important to note that the Slate command syntax may vary depending on the version and configuration of Slate being used.