If I pay off lease amount early for a car, is there any impact on my credit?
Let's say I'm paying $400/month for 36 months, which is 14,400 total. If I pay 14,400 within 6 months, are there any disadvantages on my credit?
In general the answer is yes, you can pay it off early. However, check the terms of your lease for a "prepayment penalty." They are becoming less common, but if you have such terms in your lease, you might incur a fee for paying early. Even then, if the prepayment penalty is less than the interest you would otherwise pay, it would still be worth it.
Paying off loans is a positive thing for your credit. It won't negatively impact your credit to prepay.