You should be careful about using ::: to access objects inside packages. These are objects the package author chose not to export. With a language like R, where you're free to inspect and manipulate almost everything, most protections are just "gentlemen's and ladies' agreements." One of them is that package updates shouldn't change exported functions enough to break user code. Conversely, users shouldn't expect unexported functions to work the same or even exist.
There are two ways to do your task with functions exported by forecast. First is the "standard way" of creating a model object and then forecasting with it:
hw_model <- HoltWinters(rainseriesforecasts)
rainseriesforecasts2 <- forecast(hw_model, h=8)
Next is the "shortcut" function hw that creates a model and forecasts from it:
hw(rainseriesforecasts, h = 8)