In QuickBooks Online (QBO), you can only add Terms to invoices, Christina.

To clarify, could you please provide a screenshot of the specific error you received when saving the estimates? This will help us determine the cause of this problem.

It's also important to note that adding terms to estimates is not available in QBO, as estimates are considered non-posting transactions and won't affect your accounts receivable. Net Terms are typically applied only to invoices because they represent a formal request for payment that affects your accounts receivable, however, you can save invoices without adding terms.
 

Therefore, a screenshot of the error you encountered would be greatly appreciated to help us resolve the issue and understand what might be causing it.

You can also check out this article for detailed information about creating estimates: Create and send estimates in QuickBooks.

Feel free to click the Reply button to attach the screenshot and other concerns.