You can access Sorties through the main navigation menu, as long as you have completed the War Within quest in your Codex. Once you have completed a Sortie mission, you will be able to take part in the same mission again, if you wish, but you will not get any rewards for it from the Sorties reward table.

Can you do sorties more than once?

No. You can replay them as much as you want after you finish them for loot and affinity if you want. But the end reward is given only once.

What is a sortie mission?

A sortie is a french word for “military mission.” It is defined as an operation carried out by a deployed unit, which can be an aircraft, ships or a group of people.