I was trying to compare the perfomance of different smoothing methods in the time series forecasting system (TSFS). However, I could not select the Seasonal soothing, Winter method-additive, and Winter method-multiplicative for the series I was investigating. These three methods were all grey out, I could only select the other methods.

When I tried fit models from list, I got the following notice: "error: model fitting failed. this model is not allowed for this series."

I knew that all the observations in the series should be strictly positive if I want to fit a seasonal smoothing model, and they are all positive. Anyone know what the problem is.