There are no known issues with this at this time.
To enter your teacher related expenses, open the return, type "teachers expenses" into the search box and click the "Jump to" link in the search results to take you to that area of your return. If you are still unable to enter the amount in the field, try refreshing the page or logging out and back into your TurboTax account.
Please comment if this continues to be an issue for you.