Sounds like you are mixing & matching frame rates. Your source footage then has a Motion Adapter applied to it, which won’t allow you to add a motion effect or timewarp. However, if you splice the shot into the timeline, then open the Motion Adapter (effects mode), you can promote the motion adapter to a Timewarp, and set speeds as you like.
That said, you don’t seem to be consciously working in a different framerate from your source footage, so that is something to make sure.