Thanks for trying to help, but I thought it was a given that I already have a merchant account and online payments set up. I believe most of the people here have already successfully sent invoices with the electronic payment button showing up with no problem, so your answer is not helpful at all to us as we have already done that.

WORK AROUND TO GET YOUR INVOICES OUT WITH THE PAYMENT BUTTON:

I am a complete Quickbooks novice, but I have found that in my situation using QuickBooks: Premier Plus Contractor Edition 2021 the problem is due to sending invoices out in a batch through the File/Send Forms. I only sent 72 invoices but that is apparently too many because when I send the emails one at a time using the Email button in the invoice the payment button shows up. I also went back and rechecked the Email Later button in ten invoices and then sent only these ten in a batch using File/Send Forms and it worked! The button showed up! So, sending smaller batches works although far from ideal, SO INTUIT STILL NEEDS TO FIX THIS PROBLEM! 

Hopefully this will help if you need to get your bills out asap, but it does take extra work that should not be necessary!