I had the same issue - stuck on that medical miles sheet.  I then chose the "topic list" at the top of that page, clicked on the next topic to be completed, used the 'back' button which got me to that same medical questions area.  'Continue'd thru them and when I got to the miles sheet, it had filled in the miles I had totaled on the Supporting Detail window.  I could then continue on. This worked for me, hopefully it will work for you.  I'm no expert, just someone trying to do my taxes.

STILL NOT FIXED IN TURBOTAX 2020

......but this brilliant solution works!

The 2020  Sch A is not fully operational yet ... expected to be sometime about 1/21. 

Today is 04/08/2021 ant it's still not working, but we have an extra month to get around it.

Thank you for your patience, however we are still working to finalize the Schedule A deductions in collaboration with the IRS.  We cannot make a move until IRS has finalized all of their programming then we can do TurboTax updates or changes.

Work around for MAC users: While on the Miles Driven question hit the Details option at the top left of the screen and entered the mileage there. Then click topic search to select the next screen. Finally went back to the section and the mileage was entered.