Hello Georges.
In Paratext you can have only one interlinear per language, but as many interlinears as you want as long as they are from different languages. For example, you could have a gloss interlinear with French, a retro to English, etc.
If I understand correctly, the message is because you have already configured a French-based interlinear. It would be the interlinear of glosses with LSG model text. I imagine that the same message tells you whether you want to replace it with your new configuration (NVS interlinear) or keep the previous French configuration (Glosses with LSG).
Saludos,
Pepe.