This is a good question, but it will be hard to advise you until I know your thesis. In other words, your conclusion must be in alignment with your thesis. So, if you thesis is on a particular symbol in the book, then your thesis should be on that symbol as well.
Let me give you an example of a symbol and how a conclusion can be made on this symbol. One of the most memorable symbols of the book is the chains on Marley's limbs. Marley has spent seven years roaming the earth in his chain as a punishment for his misdeeds while he was alive. When Scrooge takes a closer look at the chains, he realizes that the chains are made of cash boxes and ledgers. In other words, his greed on earth caused his punishment in the afterlife.
The symbolism cannot be missed. Scrooge's love of money is tantamount to chains. His love of money is not freedom but bondage. For this reason a conclusion should touch on the theme of bondage, freedom and Scrooge's miserliness.