Error : 'calculate_intervals' is not an exported object from 'namespace:forecastML'
calculate_intervals first came to exist on May 24, 2020 (see the history). The version of the forecastML package on CRAN is dated May 7, 2020 and does not list that function, suggesting that it does not have it. Your best bet is to either figure out how to install a development version of forecastML or complain to the author of what you're reading.