You can find the level before this on this http://www.anonhack.in/2018/09/overthewire-natas-series-level-12-14/

The 14th level of overthewire natas is a typical SQL injection based level. In this level, we have to use SQL injection tactics in order to get the password for the 15th level.

Let’s dive in:

1. This screen will appear: 
   2. If you look in the source code, it will tell you that double quotes(“) has been used instead of single quote(‘). So to inject we need a injection that uses double quote. The Injection parameter, I have used is: “OR 1=1 — –
   3. There you go, the password for the next level is right there!