When creating an IK in Blender, you need to untie the parent of the bone, which will be an IK, that is, the Hand bone needs to be untied from the Forearm bone. Thus, Unity will not be able to apply the Humanoid parameter to the rig, an error will be issued that it is impossible to bind these bones to each other, since the first is not the parent of the second. How to fix it? Ik in Blender is done according to one system, as far as I know, the Ik bone will have to be untied from the parent anyway.
By the way, about IK, what is the best way to do it? Add additional IK bones by type so that there is a “Hand” bone and an additional “Hand.Ik”, or use the “Hand” bone by itself, like IK?