철 = 계절(季节) 철 없다 itself means "to be unable to see the change of the season". So if you say 철 없다 about someone, you're saying they're childish, immature, unable to understand how things go. 땜에 is a very colloquial way of saying 때문에.

it's 철이없다. and no, 철 doesn't mean 때문에 (not 땜에). 철이없다 just means "you are immature and childlike." on the other hand when you are saying "you are mature" you'd say 철이들었다 or 철들었다.

(1)철없어原型为철없다，这里的철指分辨事理的能力 철없다字面意思为不辨事理，即不懂事 철들다字面意思为有了分辨事理的能力，即懂事 (2)땜에=때문에