the only thing your converting to a graphic is the object on the first keyframe (and that breaks the tween from the first to the 2nd keyframe).
if you're trying to convert that entire shape and tween to a graphic, copy the frames >create an new graphic > paste the copied frames on the graphics timeline. (you'll also probably want to remove the main timeline tween that's been saved to a graphic.)
@tsurielh692350 Shape tween can only be applied on raw shapes, as you are converting shape present on the first keyframe into symbol that is why shape tween is breaking as symbol can not be shape tweened.
To convert entire layer into symbol:- Right click on selected layer and then select "Convert layers to symbol" option from the context menu to convert layers into a symbol
To convert frames into a symbol:- Please follow the steps suggested by @kglad