Can I input expenses on new purchased car even if I am not done paying it?

What expenses?  Is this a car you use for business?

You cannot take any deduction for a personal use vehicle.

If this is a business use vehicle, then once you take on the liability of the loan, then it is counted as you paying for it when it comes to taxes. 

Cars are not in general deductible.  The times when you get any tax breaks for a vehicle are 

1. When you buy an eligible new or used EV and are eligible for the Clean Vehicle Credit
2. When the vehicle is used for business, you can then enter your expenses, depreciate or take a mileage deduction. 
3. You are itemizing your return and claiming the sale tax you paid as a deduction.