Hi, If the horizon is "static" but rotated then you can just use the motion tab to rotate it without using a stabilizer.
Or am I mistaking you? Lock & Load will smooth out any jitter or shake on the shot regardless and if its rotated then you should just use the motion tab to rotate the horizon back to level and zoom in slightly.
Hi, is the timeline slider actually parked on the clip that you just added when you get this error? Please add the plugin again, double click on the clip to make sure the filter controls are selected and then try and press track motion.