The "scales" R package was updated on 5/11/2020 to version 1.1.1. Currently, the version supported in Power BI Service is 1.0.0 (https://docs.microsoft.com/en-us/power-bi/connect-data/service-r-packages-support). This prevents several newly-added features of "scales" from being used, such as the label_number_si function.
I would suggest you create an idea here to let power bi product know this update. Your contribution will be appreciated.