Are you perhaps reffering to Jacob's greyed out ability? Which is in the squad screen and requires his loyalty mission to be completed before you can use it.
Regards to not being able to select things, I found that a quick save followed by quick load fixed this.
If these aren't the problem, i'm sorry, your post isn't very clear. Perhaps if you can give me more details?
This site says that the JOAB mission becomes available once you acquire (not complete!) Jacob's loyalty mission.
https://masseffect.fandom.com/wiki/N7:_Archeological_Dig_Site
There is no mention that you'd need to complete Jacob's loyalty mission to finish the mission. However, it does say that the location only opens when all enemies are defeated. Sometimes, an enemy may be hiding behind some cover relatively far from your position, preventing you from "finishing a fight".
It doesn't make sense for completion of a side mission X to be dependent on you first finishing mission Y that is not directly linked to it.
Check again.
In addition to what AlcatorSK said, I have had enemies clip through walls a couple of times, or get thrown somewhere with a biotic power that they can't get back from. So if its waiting for all enemies to be removed and you can't find them all, this could be a possibility.
Any inability to complete Joab has nothing to do with Jacob.
A 'stuck' enemy may show up on your mini-map.
Or you could simply re-load a save you made before going to Joab.
