It says it right there in Grasshopper: open a document or create one.
Either use the New command to create a New Grasshopper document.
Or if you have one, use the Open command to open it.
Those green rectangles are Grasshopper documents, you can open them by clicking on them (or maybe double-clicking, I am not sure).
Then, when you have a Grasshopper document open, it should work.
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