You are apparently running Dorico SE (the free version, without Engrave Mode) which is limited to 8 instruments, while your project has 12.
Question: did you purchase Dorico Pro or Elements (in which case your licence is not recognized for some reason)?
Il semble que vous utilisiez Dorico SE (la version gratuite), qui est limitée à 8 instruments, alors que votre projet en compte 12.
Question : avez-vous acquis Dorico Pro ou Elements (auquel cas il s’agirait là d’un problème de licence non reconnue) ?
It could also be that you held down a modifier key (by mistake) while launching Dorico. Dorico Pro can launch as SE or Elements that way.
Jesper