What do error messages mean as "model error: zero observations for changepoint at time 3"?

The time effects model requires that positive count data are available for each year. TRIM prompts this error if the model cannot be estimated for this reason. The same error may arise for a linear trend model with a changepoint selected at time point 3. When the data are missing or sparse for the third year, you should try a linear trend model with all changepoints selected except the third year.