So what you got to do is follow this guide and heres a picture of the completed picture and each number the tiles are and the unumbered tile gets added in once you finish so don’t worry about it.
Trust me this might look confusing, but if you follow the guide, it is really quite simple and you will be finished in around 10 minutes and know how to complete any 15 tile puzzle.
This guide absolutely helped, but the only problem is sometimes you can end up with this, and it become impossible to get it right without just restarting.
Luck, just luck. Solving one row at a time and making sure you don’t change it is the best decision but just try to go in and out of it till you get one that is decent.