Glad to see you here, @kwylie2005,
You're right. You must lodge the Dec-2019 BAS first. Only then the GST centre will show the Prepare IAS for that month.
Refer to the steps below to lodge your BAS in QuickBooks Online:
For more information about this process, refer to the following article: Lodging BAS and IAS.
Feel free to mention me if you have any questions about this topic or need anything else in QuickBooks. I'm always here to help. Have a fantastic day!
Selecting 'Mark as Lodged' simply closes off the tax period in QuickBooks, it does not lodge with the ATO unless you select the option 'Lodge with ATO'; you don't need to submit your BAS /IAS until you are ready to do so. In addition, if a BAS isn't actually due yet you would not have the option to 'Lodge with ATO' and can only mark it as lodged. Alternatively, you can also review your books for future periods not yet marked as lodged in the GST centre by running reports such as:
You can run these reports to your preferred date range to review your figures as needed.