Please save as TWBX or include the TDE so that we can open your workbook.
However, Tableau 8.0's forecasting feature doesn't work with numeric year fields. It only works with DATE or DATETIME fields. You can use a trendline instead.
If you really want to use forecasting (it won't find any seasonality, though), you need to use a calculation to convert the numeric field back to a datetime. Try DATEADD('year',[year]-2000,#1/1/2000#).