Hi @Dhanee,
Time to debug your code then!
Debug;
If you disable the Maximize on Play button in the Game view;
…and then run the game, then click on the Console view, this will give you an insight as to what the problem may be;

The NullReferenceException error points to line 31 in NumberWizard.cs, if we take a look at that specific line;

On this line we have a couple of possibilities… we are trying to set the text property of a UI Text object called Text and we are trying to use the ToString() method on guess.
If there had been an issue with the population of guess on the line above using Random.Range() we would expect to see an error pointing to that line, as we didn’t it would be fair at this stage to assume it is an issue with the UI Text object.
A NullReferenceException is generated when an object is referenced and doesn’t exist, so in this case we could assume that our UI Text object called text doesn’t exist at this stage in the code… Lets have a look and see if this is the case…
If we look at the top of the NumberWizard class we can see that you declare a public variable of type Text, called text, but then following the code from that point to our error, at no point do we instantiate it in any way. Thus, we are expecting it to already exist in our code, if it isn’t, then we haven’t passed it in!
We need to locate where we are referencing the UI Text object and passing it to the NumberWizard script, we know the error is caused when the Game scene is triggered, so lets start by opening the Game scene.
Looking at the objects in the Hierarchy, I found that your LevelManager GameObject has in fact got the NumberWizard script attached.

Selecting it in the Hierarchy and then viewing it in the Inspector reveals that the exposed variable called Text is not actually populated;

The resolution here would be to drag the relevant UI Text object from the Hierarchy into the exposed Text variable on the Number Wizard script, in this case, this would be the GameObject named Guess.

Having done so, this would now be the time to run the game again and check to see if this has made a difference.
Whilst testing at this end I can confirm that this does resolve the issue for the Higher and Lower buttons. However, I did note that your Correct button is not behaving as it should.
See if you can follow the same approach above and determine what the issue could be, what is happening and why? Then try to resolve it and re-test.
On a minor note - you have your NumberWizard script attached to a GameObject named LevelManager, whilst this will work, it obviously can be a little confusing, especially when you have a script called LevelManager. Once you have resolved the issues above, it would be worth considering having one GameObject named LevelManager with the LevelManager script attached to it, and another GameObject called NumberWizard and attach the NumberWizard script to it. I think you will find this a little easier going forward - as will those that help you 
Hope this helps, and let me know how you get on resolving the Correct button issue.