It seems the subject site has no label in the train_naturalized_flow.csv. I counted the number of sites in the latter, there are only 23 sites in train_naturalized_flow.csv. The subject site also does not have any streamflow related information in USGS NWIS. Please confirm. Thanks.
The problem description page indeed documents that only 23 of the 26 sites have data in the monthly time series data. The three missing sites are
It is also the case that USGS NWIS may be missing data for some USGS locations for some time periods.
As this challenge is working with real world operational data to build models for operational forecasts, published data availability is a limitation. It is possible that challenge organizers may identify additional data publishers that are appropriate for the challenge and adds them, but participants will in general need to handle missing data in their modeling.