If you entered it on your federal return, it can be reported on AR3 line 6. Although you can't claim "vehicle registration fees in Arkansas, you do claim "The Ad Valorem Tax paid to your county based on your vehicle’s value". The amount you entered for the federal return should be the same amount since the federal return only lets you deduct a tax, and a tax is only based on value. 

Please use the link below for verification:

 https://ttlc.intuit.com/questions/3926322-is-my-car-registration-fee-deductible