When you close an encounter, VetBadger will "Charge Lock" the encounter. This prevents you from adding billing items after you have closed (and checked out) the encounter.
If you want to unlock the encounter for charging, you can click on the invoice link:
This will open the Invoice Summary display.
Within the display there is a button which is a circle with a slash through it.
This button will allow you to release the charge lock and allow invoice changes to the encounter. Click this to release the charge lock and make any additions you need to the encounter.