You may have forgotten to accept the mission. If you accept any mission it will appear on the second tab, "Transactions", on the left side menu. Check missions again, and it may populate again for you. Otherwise, you can accept any trading missions (albeit, read the risks, rewards, and requirements before doing so) and you will learn how to trade. As the game itself has no core story line, you are not gimping your experience in game. This game is one of those, "blaze your own trail", type games so you make what experiences you want out of the game. Whether you decide you want to trade, mine, bounty hunt, support major or minor factions, engage in civil war/war combat, engage in piracy, explore, participate in power play factions, smuggle, or a combination of many of these options, among others, it is entirely up to you. i would just recommend that you stay near or about the starter area until you get more acclimated to the game, and have the funds to purchase a better ship with an improved jump range. You should also check your coms menu, which is the upper left hand corner. The mission acceptance and brief may still be in that window (I believe the second to last tab in the coms window to the right side). If it first appeared in that window and tab, and you didn't delete it then it should still be in that location.