In quickbooks 2019 desktop pro I could go to customers, then click on a customer to see invoice email history. cannot find that in quickbooks online version

I see you want to view the history of your emailed invoices. I'd be happy to walk you through the steps to achieve this.

You can open the invoice and go to the Audit History. It will show the date and time you emailed an invoice to your customer.

Here's how: 

1. Login to your QuickBooks Online. 
2. Go to the Sales tab and then select the Customers section.
3. Click the name of the customer whose invoice you want to know the email history.
4. Search and select the invoice from the list. 
5. From the invoice page, choose More at the bottom and click Audit History.
   
   

I'm adding this article you can use in the future: Customize invoices, estimates, and sales receipts in QuickBooks Online.

That's it! Let me know in the comment section if you need more help managing invoices in QuickBooks. I'll be around to help. Stay safe!