Forecasting settings are a tricky metadata type as to successfully deploy them, 4 separate metadata types are required.

Make sure that in both your source and target, forecasts are enabled.

In your metadata filter you will need to include Forecasting source definition, Forecasting type , Forecasting type source and Settings.

In this example, I have the forecast type Gearset Deployment

Once I have run the comparison with the selected metadata above, I'll need to select Forecasting settings, the related Forecasting source definition, Forecasting type and Forecasting type source

Note: To ensure you are selecting the correct Forecasting source definition. Go into the Forecasting type source and read the XML in the XML viewer.

Once all of these have been selected we can move through the deployment steps and complete deployment.