I know how to fix the error so you can edit and save the quote, mastermind.
You may run into this error on the mobile Android or iOS app if the Terms list in QuickBooks has not been populated by default or has been deleted.
Let's sign out of the QuickBooks app on your mobile device and perform the following actions by signing into QuickBooks on your computer browser.
Click the gear icon on the top right and select Company Settings.
Choose All Lists, then Terms.
Click New.
Enter a name for the term.
Choose Due in fixed number of days and enter 0 next to it.
Click Save.
Log out of QuickBooks on your computer.
Thereafter, log back into the app on your mobile device and modify the quote once more. You should no longer receive the error. I'm adding this article to help you manage customer's transactions: Create and send quotes in QuickBooks Online.