This typically happens if you have already applied a Motion Effect to a clip. You cannot apply motion effects on top of motion effects, so you will have to create a Video Mixdown of your clip to apply another Motion Effect. This can be done by dropping your motion effect clip to your timeline, then selecting Video Mixdown from the Clip menu.
Select the bin you wish to create the Video Mixdown to, the drive you wish to store it on and the resolution you need and click on OK to create the mixdown.
You may then open your new clip and apply another Motion Effect.
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