What dates/period? Maybe there are works along the line that time.

Though most likely: its INTernational and for most such trips it simply cannot be done directly online, or at least via app. Maybe via b-rail, maybe by (also cutting the cost a lot) taking a local train a few mins earlier and board TGV in first FR stop (Metz, maybe Thionville).

BTW-IF ou stay in LUX before there: all local transport, bus/tram/train is FREE in this country-only domestic!!.

Every date I looked for in June. 

Couldn’t find a way to reserve on b-rail. 

But it’s strange, how can be the train unavailable in every date I searched lol?

1. The Interrail website can make reservations for international TGVs.
2. The rail planner app does not do reservations, it just forwards to websites.
3. B-rail only does Thalys/Eurostar and domestic reservations in France.

This particular train is a bit strange indeed. The reservations are open but interrail.eu doesn't seem to be able to book from Luxembourg to Paris. In the other direction it works correctly.

The best option is then to book at a station in Belgium, France, Luxembourg or Switzerland or to call SNCF (press #85 for English). There should be no booking fees for any of these options (Interrail charges 2€ per person and seat).