Train supervised classification algorithm on multiple Sentinel-2 tiles (i.e. multi-temporal classification)?

I believe the answer I’m looking for is here: Multi-temporal Random Forest - NaN pixels

More details here: https://rus-copernicus.eu/portal/wp-content/uploads/library/education/training/LAND01_CropMapping_Seville_Tutorial.pdf

However, when I try to run the classifier I get the following error:

Error: [NodeId: Maximum-Likelihood-Classifier] Cannot select feature band B1 in more than one product

I’m using SNAP version 6.0.4