It looks like Tableau could not find
any patterns in your data
good enough to forecast.
For monthly seasonality, one needs
at least a full 12-month period of data,
better to have a full 24 months of data.
For a quarterly forecast one needs 9 consecutive quarters.
If what you see as a forecast doesn't make sense,
you may want to look at the Forecast Options (to change some)
and at the Forecast Description (to see the quality and the factors
of the chosen model).
Hope it makes sense.