For 1099-K, TurboTax asks me to "double click to select the form on which to report this income." Double clicking on Schedule C does nothing and error persists.

If you're getting an error, just enter the 1099-K income directly in Schedule C. 

You can enter the income reported on your Form 1099-K (along with any cash or checks you received) as self-employment income on Schedule C. Here's how:

1. Open or continue your return and select Search
2. Search for Schedule C and select the Jump to schedule c
3. Answer the questions about your business until you get to the Here's your info screen. Select Add income for this work
   1. If you’re returning to Schedule C, select Review or Edit next to your work and then answer the questions about your business
4. On the Let's enter the income for your work screen, select Other self-employed income and Continue
5. On the Tell us about other self-employed income for your work screen, enter your 1099-K info
6. Select Continue when finished

If you are entering income from a 1099-K, remember that it reports the gross amount of the transactions. It doesn't include any adjustments for credits, refunds, discounts, or fees. 

• You’ll have a chance to enter self-employment expenses in another part of your return.