How to fix C++ error: expected unqualified-id

Semicolon should be at the end of the class definition rather than after the name:

class WordGame
{
};

Get rid of the semicolon after WordGame.

You really should have discovered this problem when the class was a lot smaller. When you're writing code, you should be compiling about every time you add half a dozen lines.

There should be no semicolon here:

class WordGame;

...but there should be one at the end of your class definition:

...
private:
    string theWord;
}; // <-- Semicolon should be at the end of your class definition

As a side note, consider passing strings in setWord() as const references to avoid excess copying. Also, in displayWord, consider making this a const function to follow const-correctness.

void setWord(const std::string& word) {
  theWord = word;
}