want to know mistake in the following code

There are two issues:

1. In the addTile method you wrote in task 1 you are adding the string "tile" to the tiles variable instead of adding the contents of the tile variable like you are meant to. You are only meant to use double quotes when you want to create a string, when referencing a variable you should just type the name of it.
2. The code in the hasTile method you wrote in task 2 is technically fine as it would actually work correctly, but it is inefficient to use an if statement when the expression you have written returns the right Boolean on it's own. You can return the expression directly, which is more efficient and is what the task is looking for.

If you fix those two issues like this:

public class ScrabblePlayer {
  // A String representing all of the tiles that this player has
  private String tiles;

  public ScrabblePlayer() {
    tiles = "";
  }

  public String getTiles() {
    return tiles;
  }

  public void addTile(char tile) {
    // TODO: Add the tile to tiles
    tiles += tile; // Remove quotes around the `tile` variable
  }

  public boolean hasTile(char tile) {
    // TODO: Determine if user has the tile passed in
    return tiles.indexOf(tile) >= 0; // Return the result of the expression directly
  }
}

Then your code will be accepted.