I just go the mission "Dekuuna: Elcor Extraction" and I need to go to the planet Dekuuna. The mission does not specify where I should go to find this planet and I don't see any indication anywhere on the galaxy map of where I should go. So, what mass relay and system should I go to in order to find Dekuuna?
The Elcor homeworld of Dekunna is in the Silean Nebula, in the Phontes system. I see that you've found Oltan - Oltan is actually a satellite of Dekuuna, so it should be right there.
If you found the Elcor asset there, you're in the right place - the ships you're looking for are apparently on Oltan, actually. (It's even possible you already found this one without realizing it, maybe)
The planet with the survivors you're looking is named Oltan. Oltan is in the Phontes System which, in turn, is in the Silean Nebula. As for where Dekuuna is, it's the big planet right next door to Oltan.
Once you scan Oltan and pick up the Elcor Flotilla, just head back to the Citadel Embassies and talk to the Elcor Ambassador again to finish the quest.