Sophia here from Shopify. Welcome to the Shopify Community!
You're correct that the 1099-K form for Shopify Payments will be under Payouts > Documents. Are you using Shopify Payments exclusively, or are there other payment gateways like PayPal or manual payment methods enabled on your store? The 1099-K form is available when the total processed through Shopify Payments is $20,000 or higher and there are 200 or more transactions, so if orders came through outside of Shopify Payments, they wouldn't be included in these totals.