Esri's tools and data model may have stricter requirements on the GTFS fields than the GTFS specification itself, so your data might pass the validator but still not be usable for creating a network dataset.
Regarding that specific error about invalid non-numerical values, another user reported a problem recently where they were getting this message when they shouldn't. It was because their trips.txt had a bunch of tabs in it, and the tool code was incorrectly reading the value as "[tab]1" instead of "1" and failing to strip off the stray tabs. This bug has been fixed in the forthcoming Pro 3.3 release. But anyway, it's worth checking if your data has stray tabs in it and getting rid of them if it does. (Tabs can often get added to CSV files if you open them in Excel using the wrong delimiter and save them back out.)