If you are tracking an object that moves out of frame then the mocha tracker will do it's best to keep tracking the shape as long as possible. If the track shape moves completely off screen (out of frame) then the tracker will stop, this is normal, as there is nothing it can track.
What you then need to do is draw new track shape on screen on an object that is moving in the same direction and in the same perspective plane as the object that went off screen. Then press the button to track again, the track will continue using the track data up to the point where it stopped. In some case you may need to delete some bad track frames or add a few keyframes to get the best results.
Please see this tutorial:
If you get this error many times, then it's likely that you have choosen something which can't be tracked well, this usually means it's got strong reflections on it, is a uniform constant color, has constant movement (eg like rippling water) or is not a planar surface. Please see this video which explains how to deal with this and choose a better track area.