As Raildex points out, you need to create the asset, first. This is actually stated quite clearly, in the tutorial you have linked.
It appears likely that you have mistakenly skipped a part of the tutorial. You mention being told to locate
Aim_Center, which appears to be section three, but only finding
Aim_Offset, which you would have made in section two.
Aim_Offset is being made in section one. I feel this is very clear, though it is referenced as
Aim_Offset. This should not cause any confusion, if you are following the tutorial. This is still consistent with the name used, later, when you are asked to find it.
It is important you follow the entire tutorial; start to end. In this case, the missed section makes up more than the
Hero_AimOffset asset; it is likely that if you missed one, you probably missed more.