You can show a financial hardship (for example you have very large medical bills or some other difficult circumstances)
Applying for a Waiver
If you want to apply for a fee waiver, read the instructions and fill out the I-912 form. Send it with proof of your income (the instructions tell you what you need) in the same envelope with your citizenship application.
If USCIS approves your request, you will get a receipt notice that says the "filing amount" is $0. Your citizenship application will be processed in the normal way.
If USCIS does not approve your request, they will send your citizenship application back to you. It will come with a note that explains why they did not waive the fee. You can try applying for the fee waiver again, or you can send the citizenship application back with the fee.