If the installation was completed in 2018 then you have claim the credit on the 2018 tax return, not on the 2019 tax return. The 2018 tax return that was filed will have to be amended to claim the credit.
See this TurboTax support FAQ for amending a 2018 tax return originally completed and filed using TurboTax - https://ttlc.intuit.com/community/amending/help/how-do-i-amend-a-return-in-turbotax/01/27579
Assuming the solar panels were installed on your primary residence, you can claim the energy tax credit in the tax year those solar panels were placed "in service" and started producing electricity. If that was in 2018, then you'll have to amend the 2018 tax return in order to claim the credit.