The reason why you can't select a Standard/Advanced plan during a purchase (while the Premium plan is available) is that your Organisation is not connected to the accounting system (Xero or QuickBooks Online).
To establish a connection between your ApprovalMax Organisation and an accounting system, follow the steps below:
In the main menu, select your Organisation from the drop-down list
Navigate to Workflow and settings
Open the Organisation Settings tab
Look for the appropriate accounting system and the CONNECT button next to it
Click on CONNECT to initiate the connection process