try rendering out the clip with lots o fextra time at the end, as long as you need it to be then re-import it.
If you use any time changing effects like time remapping you can and you have to extend the out point of the video.
If you use Time Warp or several other time manipulating effects you have to pre-compose the original clip moving all attributes into a composition that is long enough to fit the entire shot after the speed is changed, then apply timeworn to the pre-comp.
IOW, if you have a 6 second clip that you want to time warp to a speed of 50% then you need to put the 6 second shot in a 12 second comp, pre-compose the layer moving all attributes to the new comp, then apply Time Warp to the new layer.
If you had typed time warp in the search help field at the top right corner of AE you would have found instructions and tutorials that explain exactly how to use the effect.