No, you cannot claim a capital loss from 2019 on your 2020 tax return. You can amend your 2019 tax return. To be clear, a reverse split does not create a loss. For example, if you owned 100 shares of a stock with a cost basis of $1 per share, and then there was 1 for 5 reverse split, you would have 20 shares with a cost basis of $5 each. You may have reported the wrong cost basis if you didn't use the updated cost basis when you sold it.
To amend a 2019 return in TurboTax, see How to amend (change or correct) a return you already filed
No, you cannot claim a capital loss from 2019 on your 2020 tax return. You can amend your 2019 tax return. To be clear, a reverse split does not create a loss. For example, if you owned 100 shares of a stock with a cost basis of $1 per share, and then there was 1 for 5 reverse split, you would have 20 shares with a cost basis of $5 each. You may have reported the wrong cost basis if you didn't use the updated cost basis when you sold it.
To amend a 2019 return in TurboTax, see How to amend (change or correct) a return you already filed