A Sequence animation is an animation that uses a picture you used in original animations, but you can select the frames used in the sequence & their order. Example: you couls put all the animations of a character in one file, and separate them into the separate anims you wanted (walk left-right stand left-right etc.)
Back to the point:
My first idea
was that you had already created the actor, with that animation, but after I've tested I realized that can't be. So I think the problem is not with that it's a sequence, because that's recognized as an animation too. I've no idea...