What does this mean: The changes made to invoice needs an internal approval before it can be sent to customer?

You'll receive the internal approval message once a common task and reminders are set up or configured on the Workflow automation page. I'll guide you on how to sort this out, Brenda. 

When setting up a task, you can find it under My Workflows tab on the Workflow automation page. I've added screenshots for your reference guide: 
 

This setup provides convenience for account administrators by enhancing security and restricting entries in QuickBooks, which ensures the accuracy of your financial records. This is the reason why you're getting the error: 
 

When you click the "Submit for Approval" button, you'll notify the approver or validator about this transaction. Using workflows, you can automate routine tasks and notifications such as invoice reviews, payment collections, and transaction approvals. You define the criteria for when reminders should be sent and to whom.

However, if you don't want this message to appear, you'll want to reach out to your Primary Administrator and inform to disable the feature. Just toggle it to off to disable it. 

Please know that you'll have to approve all open invoices before your can disable this workflow. 

The screenshot provided maybe from the US region but it functions similarly in the Canada version.

To give you more details on how to set up the workflow, I've added this for you: Use workflows to automate your business processes for QuickBooks Online Advanced.

Additionally, learn how you to record invoice payments with QuickBooks Online: Record Invoice Payments.

If you have any questions or other concerns when creating invoices and workflows, let me know. I'm always right here willing to assist you further. Have a nice day!