can only be; no alternative, only ~
Expresses that there is no alternative except the stated person or thing; 'no one/thing but ~'.
This is a formal, emphatic way to say that something is possible only with the noun you name. It's like saying 'apart from ~, there is no...' or 'if not ~, then no one/thing else can.' The noun is usually a specific person, group, or time, and the negative clause completes the idea of exclusivity.
You'll often see it in set phrases like 彼をおいて他にいない (there is no one else but him) or 今をおいて絶好の機会はない (there is no better chance than now). It carries a strong, almost dramatic tone, so it fits speeches, essays, or formal conversations.
Because it's N1 and very literary, you won't hear it in casual chat. If you use it with friends, it might sound like you're quoting a novel or making a joke. Stick to contexts where you want to sound decisive or solemn.