I called their support number and they had no clue there was an issue.   Took them 30 minutes to acknowledge the program wasn't ready.   Total chaos.   What's worse is that the website still sells you the product even though they know it's not complete.   At a minimum they should just list the forms that are not ready ... like the Mortgage Interest and Schedule A.    

If you click on Review in Step by Step and work your way through it, you will come to a screen that lists forms that are not yet available and when they are expected to be available. The availability date for Schedule A which is where medical expenses are entered is 1/17/18. The IRS has not yet completed final versions of many of the forms in the tax return. Until those forms are finalized and TurboTax implements them into the program and gets the IRS approval for the implementation, the program will not be finalized.

Just ran into the same problem, except the program would accept a number for "Other (taxis, etc)" transportation expenses. Go figure.

I reported this problem for the 2017 return to TTX support.  They said it would be corrected.. apparently the fix was overlooked again for 2018 tax year program.

It is February 7 and is still an issue.  When will this be corrected?

It is February 24, 2019 and it still does not work

This is BS. And my last year for TT.

today is April 3rd and the program will not allow me to enter medical miles driven

This seems to only be a problem with certain installations of the program. If you are using the CD/download product, have you tried entering the mileage directly on the medical expense worksheet? Have you tried double clicking the box where you enter the mileage?

Your work arounds are correct. But the program is inconsistent. other sections of the program work the same way they have for years in the use of supporting worksheets when using the step by step method.