On the desktop version, this happens to me when TT wants to open a worksheet behind the entry. For example, when I want to enter the total amount I spend on medicine, TT wants me to list how much I paid to each pharmacy. There is no message indicating this but I found that if you DOUBLE CLICK the field, it pops up the worksheet. This is a very bad design IMHO with a very bad user experience. But try double clicking.
