"Can't find suitable initial frames"

If you encounter the message "Can't find suitable initial frames", this indicates limited perspective in the shot (or some bad trackers).

Turn on the checkboxes next to Begin and End frames on the Solver panel, and select two frames with many trackers in common, where the camera or object  rotates around 30 degrees between the two frames. It's also a good idea to turn on the "Slow but sure" checkbox in this situation.