New Player - Mission is "complete" during execution, but not after returning to Kerbal

[taghid]

I have accepted a mission to "Test LFB KR-1x2 Twin Boar Liquid Fuel Engine orbiting Kerbin".  The requirements of the contract include reaching orbit around Kerbin and an altitude of 530km to 540km.

Twice I've had all of the objectives met during the mission and everything is marked "green", yet when I return to Kerbal, the mission is incomplete.

I am obviously missing something, but I don't have any idea what it might be.

[Starman4308]

There's two categories of missions involving parts. The first is a "haul part to X situation" contract, in which case all you need is to have it onboard.

What you likely have is the "test part in X situation" contract. For this, you either need to right-click and click "test part", or use staging to activate the engine, mostly likely the latter for engines. And yes, this means hauling the Twin Boar, an engine clearly intended for booster work, all the way to orbit before activating it.

[EpicSpaceTroll139]

An add-on to @Starman4308's post. While you may have to activate the twin-boar in orbit via staging, it does not matter if it is empty of fuel. You can even have used it as part of your rocket in the launch by activating it by right click menu, just as long as you don't stage it.

[Kryxal]

For that matter, it's not too difficult to stage the engine, then rearrange your staging and stage it again when conditions are met.