I'm going to assume that you are certain that you need to fill it in, and will describe my recent experience and solution.
I had the same situation, and it was pre-populated with 0 (zero). After working with tech support extensively, we ultimately found a workaround. I deleted that vehicle and added it back in. Then I was able to enter all of the correct data. It seems to me like when the vehicle was transferred from my 2017 taxes, something was not set correctly, which caused this situation. Replacing the "transferred' vehicle record with a new one solved the problem.
Hope this helps.
