If you can't use the credit because you don't pay enough tax, there may be some ways to temporarily increase your tax bill so you can use up the credit.
If you are contributing to a pre-tax retirement plan and getting a tax deduction (IRA, 401k, or similar plan) you could switch over to making non-deductible contributions to a Roth IRA or Roth option account in a 401k or 403b. That will raise your taxes, and by putting the money in a Roth-type account, it will grow tax free and be tax-free when you withdraw it in retirement. For example, if you are in the 22% bracket and your credit is $5000, you could contribute about $22,000 to after-tax Roth accounts to create a tax bill to use up the credit. (That could be $6000 per spouse over 2 years.)
You can also do a conversion of existing pre-tax balances into a Roth account, which creates a taxable event that you can cover the with credit.
Another idea would be to sell some investments that have gained in value to lock in the gains and pay no tax because its covered by the credit.
You would really need a financial advisor to look at your tax return and your family finances to see exactly why your tax is being zeroed out without the credit, and what your best options are for temporarily increasing your taxable income so you can use up the credit and get the benefit of some tax-free investments or other transactions.
