If you guys are doing everything from MongoDB Compass like I was, then probably the critical mistake is the field, which you dedicated to be a timestamp when creating the collection.
As described here, while creating the collection you must specify the fieldname for the timestamp: https://www.mongodb.com/docs/compass/current/collections/time-series-collection/
Unfortunately after the creation this property is no longer visible (or at least I was not able to find it). If the the timestamp column in your input CSV file has a different label compared to what you have specified, Compass will complain with this error. It took me a while to figure this out and I confirmed it by creating a new collection. Hope it helps!