Hi Chris,

Thanks for posting in the Community!

You can turn on the custom fields option to add internal notes if you're subscribed to the Essentials and the Plus versions. If not, you'll have to upgrade your subscription to either Essentials or Plus.

Here's how to add custom fields in your sales forms:

1. Click on the Settings icon.
2. Click on Custom fields.
3. Click on Add custom field.
4. Enter a name in the Name field, like Internal notes.
5. Don't turn on Print on form since this is for internal information only.
6. Click on Save.

Once done, open an invoice you've created and locate the Internal notes field to enter information and click on Save after.

For reference you can also check this link to learn more about: Add custom fields to sales forms and purchase orders in QuickBooks Online.

Drop by again in the Community if you have other questions. I am around to help you.