Like many other weapons in Warframe, to obtain the Rubico you have two options. The first one is to buy it for 225 Platinum in the Market Console or directly through your Arsenal. En both situations you obtain this weapon already crafted in your inventory with an Orokin Catalyst installed, as well as an extra Arsenal slot.
The second option is to buy the Rubico’s Blueprint in the Market Console, to after craft it in the Foundry of your orbiter. This will cost you resources, which you can obtain during your missions, and it will also cost you some time until you can claim it.