I'm not able to modify the date an invoice was paid and this is severely affecting my ability to complete my 2019 taxes. Is there someone on the Intuit team who can help?

Hello there, @jcaviness2019.

You have to mark the paid invoice as unpaid so you can modify the date of the transaction. Let me walk you through how: 

1. On the left pane, select Invoices. 
2. In the Invoices window, find the invoice you wish to edit.
3. Under the ACTION column, click the drop-down and select Mark as unpaid. 
4. Click the drop-down again and choose Edit. 
5. Change the invoice date, then click Send invoice.
6.  In the Recipient's email address field, you can enter a dummy email so it will not be sent to your customer again. 
7. Hit Send. 

After that, you can change the invoice status to paid again by clicking the Mark as paid link. 

I've also attached some articles which you can use if you wish to let your customers pay their invoices electronically and some FAQs about the product:

• Online Payments

• QuickBooks Self-Employed Overview

Keep me posted if you have any other questions. I'll be happy to help you out.