Dreaming of a piano synthesizer keyboard, especially in the context of feeling frustrated about forgetting to purchase it, may reflect a few underlying themes in your life.

1. Creativity and Expression: The synthesizer keyboard represents creativity and self-expression. You may be feeling a desire to express yourself or explore your creative side but feel hindered by something you believe is missing in your life.
2. Awareness and Preparation: The frustration over forgetting to buy the keyboard could indicate that you are feeling unprepared or unaware of certain aspects of your life that you need to address. It might signal a need for greater awareness of your goals or the tools necessary to achieve them.
3. Completion and Fulfillment: The idea of needing the keyboard to complete a setup suggests that you may be seeking completion in some area of your life. This could relate to personal projects, relationships, or even goals that feel unfinished.

Overall, this dream encourages you to reflect on what you may be overlooking in your waking life and to consider how you can better equip yourself to pursue your creative endeavors or fulfill your goals. Taking time to reassess your current situation and what you need to move forward could be beneficial.