you start with all the cells being inside the loop you need to click with a mouse left button or touch on the edge tiles of the loop to exclude them, if thats not working then it could be another problem.
yes, the solutions are not unique, it would be very hard to make sure the puzzle has a unique solution probably requiring a brute force algorithm, the puzzle itself is NP complete, the way I generate the puzzle is to first create the "loop" with all cells having numbers in them then removing numbers from cells at random.
I should add a tutorial with images because a lot of people were commenting thats it's not very clear how the game should be played, thanks for not giving up trying to understand it!