When creating sketches in Inventor you need a flat surface or face. When starting off a new part file you will have access to the origin workplanes. Once the first sketch and feature have been created, you will have additional faces (if they are flat not curved) to create additional sketches. If you need to create additional workplanes at different angles or positioned away from existing geometry you have workplane tools that allow you to create additional planes off those existing features and planes.
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