You should be able to create and save the invoice.
Let's open your QuickBooks Online account in a private window to isolate the issue. This doesn't store additional cache file that may be causing the initial error, allowing you not to save the invoice.
Here's how to create an invoice:
Select Create ⨁, then select Invoice.
From the Customer ▼ drop-down, choose the name of your customer.
From the Terms ▼ drop-down, choose the term when you expect the payment from your customer.
Check the Invoice date and the Due date.
Under the Product/Service column, select the item or service that is being billed, then complete the rest of the fields and other optional information.
Once your customers paid their invoices, you can enter the payment in QBO. Feel free to read through this article for the detailed steps: Record a Customer Payment. This link also provides the steps on how to record partial and joint payments.
If you're getting the same result, please let me know so we can try other troubleshooting steps to keep this working for you.