This could have a multitude of meanings. Duolingo would do well to give 2-sentence examples to add a little context. 없어질꺼요. could be in regard to cookies on a plate. 누군가 머어버릴 터인데
여긴 남아있지 않을꺼요. might be in reference to a table in a restaurant. It's too loud. They won't stay here. It could be in reference to footprints in the sand, somethings in a precarious position that might come loose or fall over, or it may be a command to move something. "Those dirty boots will not stay here." It could also mean 그들은 이동내에서 오래 안살꺼요.