Can I supply sectors with strategic resources?
Solution 1:
You need to add a system with the resource to the sector in order for the sector to have access to it. Your core planets have access to any resource in your core sector, which is all the systems not in a sector.
So, in your specific situation, you can either remove the system from the sector so that both it and a system supplying the resource are in your core sector, or you need to add a system supplying the resource to the same sector as the system trying to consume the resource.