Are you sure you’re using the correct user ID when you’re access the datastore? You could try printing out the user ID to make sure it’s correct.
Also keep in mind that if you’re running a test server, the user ID is going to be really short, so if you’re getting strange results from the print that’s why. It shouldn’t affect the results.
If the user ID is correct, the problem may be that the server is shutting down when you leave the game. If that’s the case, this article is very helpful. (I did not write it.)