I got a 2020 1099-G from CA, Box 2 is for tax year 2018 BUT Turbotax is not giving me the option to enter Box 2 when I enter a new 1099-G.

Since your 2020 form states it is for tax year 2018, it probably represents a state tax refund from 2018.  

Use these steps to get to the right place to enter your information:

• On the top row of the TurboTax online screen, click on Search (or for CD/downloaded TurboTax locate the search box in the upper right corner)
• This opens a box where you can type in “state and local tax refund” and click the magnifying glass (or for CD/downloaded TurboTax, click Find)
• The search results will give you an option to “Jump to state and local tax refund”
• Click on the blue “Jump to state and local tax refund” link