Hello Amir,
Continuous futures are not tradable assets, they simply provide a reference to the chain of consecutive contracts for a given underlying commodity or asset. To place orders, you will need to query the primary contract that the continuous future is currently pointing to. You will need to do the following:
primary = data.current(context.gc_future, 'contract')
order(primary, 1)
If you haven't already, I would recommend you checking out the Getting Started with Futures tutorial. It covers the basics of the Futures API and walks you through writing your first pairs trading algorithm.
I hope this helps.
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