be not cut out for something 
 
If someone is "not cut out" for something, such as a specific task, skill, or job, it's a gentler way of saying that they're not good at it. It means that the person is not the right type to perform the deed.
 
如果某人not cut out for某事，比如某项特定的任务、技能或工作，这是一种表示他们不擅长这些事的委婉说法。这意味着这个人不是执行该行为的合适的人。 
 
“天生就不适合…”
 
如果本人自己用了这个短语可能是一种自嘲的说法，但是如果是第三方说的就没有自嘲的意思。
 
I'm not cut out for an office job.
我根本就不是坐办公室的料。
 
I'm sorry, but I quit. I'm just not cut out for dealing with customer complaints.
对不起，我辞职。我生来就不是处理客户投诉的料。