It looks like GloFAS has been moved from the Climate Data Store to the Early Warning Data Store. Version 4.0 of the dataset as well as legacy versions (i.e 3.1) are available here: River discharge and related historical data from the Global Flood Awareness System
Note that if you download this dataset using the cdsapi tool, you’ll have to change your URL in your .cdsapirc file to https://ewds.climate.copernicus.eu/api to download the dataset