N1
てはかなわない
It's unbearable when, It'd be unbearable if, I can't stand it when, I couldn’t stand it if
今繁忙期だしこんな時にやめられてはかなわない。
It’d be unbearable if they quit during peak season.
Explanation
「〜てはかなわない」 means 'I can't stand it if this goes on'. Constant heat, noise, hassle or someone's selfishness wears you out, and you complain: 「毎日こう暑くてはかなわない」 (I can't stand it being this hot every day), 「隣がこううるさくては、仕事にならなくてかなわない」 (With the neighbors this loud I can't work; it's unbearable). It is only for unpleasant things, never for good ones. In speech it often shrinks to 「〜ちゃかなわない」 or 「〜じゃかなわない」.
Mixed up with
「〜てはかなわない」: can't take an annoying outside burden; always a complaint, never for good things.
「〜てたまらない」: a very strong bodily feeling or desire, good or bad, e.g. 「喉が渇いてたまらない」, 「会いたくてたまらない」.
「〜てならない」: a thought or worry that keeps welling up by itself, e.g. 「気になってならない」.