Hello, please why Can't i modify my invoice or sales receipt?Why does the total amount is not appear after adding payment in the invoice or sales receipt?

Hello there,  @dmsmedicalequipm.

I'm here to help you check the reasons why you can't modify invoices or sales receipts in QuickBooks Online (QBO).

I've checked here if there are ongoing issues about this, and so far, there aren't any. The initial troubleshooting steps we can do is verifying whether or not your browser's causing this to happen. 

A typical browser uses a cache of memory to work efficiently and load pages faster, but as data collects it can cause unexpected results. To start with, let's log in to your QuickBooks Online account using a private browser and send the invoice back to the same customer. 

Here's how:

• Google Chrome: press Ctrl + Shift + N 
• Mozilla Firefox: press Ctrl + Shift + P
• Microsoft Edge: press Ctrl + Shift + P
• Safari: press Command + Shift + N

If it works, you have to clear your browser's cache to have a clean slate. If not, I recommend using another supported browser.

Additionally, I've also included this reference for the different methods you can use in accepting payments from your customers: Record invoice payments in QuickBooks Online.

If there's anything else that I can help you with editing your invoices and sales receipts, please let me know by leaving any comments below. I'll be here to lend a hand.