How to deal with negative forecast values in time series forecasting? Data - Monthly Rainfall of a region for the past 20 years
Objective - To Forecast for the next 2 years
I have used a SARIMA model and predictions have been done using R. but one of the forecast value is negative. how to tackle this problem in R? as it is rainfall forecast it cannot be in negative.

In cases where the outcome has to be non-negative, it is common to model the logarithm $z=\log(y)$ instead of $y$  and then predict using $\exp(z)$ (which is guaranteed to be non-negative).
In your setup, I think you may find an additional problem: that in some periods rain might be zero. This would certainly be a problem with daily data;  it might be less so with monthly data.
