If you can't select the Standard or Advanced plan during a purchase - while the Premium plan is available - it means that your Organisation is not connected to the accounting system (Xero or QuickBooks Online).
This is how you establish a connection between your ApprovalMax Organisation and an accounting system
In the main menu, select your Organisation from the drop-down list
Go 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: