undefined index

I am a total beginner, and was trying to get the name using this code.

Document

header {

   background: #e3e3e3;
   
   padding: 2em;
   
   text-align: center;
     
   }
 
 
 </style>

Level 122

Please format your code by putting 3 backticks ``` on a line before and after each code block

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