The amount I am entering into the Solar Electric Costs field doesn't seem to get retained by the software. I have a tax liability, so I know I qualify for the credit. Is this a bug? (TurboTax Online)
Stop going back to that section and the amount will not disappear. Now for the credit ... did you enter in all your income yet ? Until you do the credit will do you no good. If you have entered in the income then review the actual tax forms to see how the credit is being computed and why none is deductible this year.
You can view your entire return or just your 1040 form before you e-file: