Can I import CSV file of crypto transactions for amending 2018 tax return (I can't find this option in TurboTax)?

You might be best advised to contact Support (via the link below) at this point.

https://ttlc.intuit.com/community/using-turbotax/help/what-is-the-turbotax-phone-number/00/25632

A CVS file cannot be imported ... 

How to enter a summary in lieu of individual transactions

Your brokerage statements should include a summary of your transactions, grouped by sales category, for example "Box A short-term covered" or "Box D long-term covered". You'll enter the summary info instead of each individual transaction.

1. Open your return in TurboTax Online (if it's not already open) and search for stock sales in the upper-right corner.
2. Click or tap the "Jump to" link in the search results.
3. Answer Yes to both Did you sell any investments? and Did you get a 1099-B?
   • If you land on Here's the investment sales info we have so far, select Add More Sales. Then answer Yes to Did you get a 1099-B?
4. On the Choose Your Bank or Brokerage import screen, select I'll type it in myself.
5. Select or enter your brokerage on the next screen and continue.
6. On the following screen (Tell us about...) select the second option, I'll enter a summary.
7. On the same screen, enter the total proceeds (sales) and cost basis, along with the sales category. Refer to your 1099-B for the amounts and category. When finished, select Done.
8. Follow the onscreen instructions to enter additional sales or to view a summary.

When you are done entering all your sales transactions, you'll eventually come to the Here's the investment sales info we have so far screen. Here you can edit, delete, or enter more sales.