For an Aim Offset, your Up, Down, Left, and Right “animations” need to just be 1 frame, with the character (in this case the turret) posed at the extreme Up, Down, Left and Right positions respectively.
The best way to do this is to create 1 animation that demonstrates the full articulation of your turret. Then capture a frame at each one of the extremes and blend them together. Also, you will probably want to add 5 more poses so your list is like:
DownLeft
DownCenter
DownRight
CenterLeft
Center
CenterRight
UpLeft
UpCenter
UpRight
Then blend between all of those. I highly recommend this page https://docs.unrealengine.com/latest/INT/Engine/Animation/AnimHowTo/AimOffset/index.html
Run through this tutorial and don’t skip any steps, no matter how small. It’ll give you a pretty good understanding of how to create an aim offset.