I don't know about other difficulties of Sudoku on the phone but sometimes on pro difficulty games start bugged and are unable to be completed, here is a video of some of the times I find out that the game session is impossible to complete.
It does not allow you to place a number in that cell because all the numbers in the row and/or column have been used already, which then disables the use of another number.
Just try and figure other ways to find the solution to it.
This isn't a bug, just a minor error on your part.