Opening the online playable version or opening a downloaded version?
For offline, make sure you've unzipped the downloaded file and you should just need to click the .html file to start the game.
If this isn't the solution, I'm going to need some more information and/or screenshots to work out what the issue is.
Since I don't play the game on mobile I'm not entirely sure of the process, but from what others have said you need to access the game through your devices file explorer. It works similar to a file explorer on PC, and has a few more options available. I believe a few previous comment threads on here have more detail about how they were able to get it to work.
