The reason you cannot add CPT codes/Charges to an encounter is typically because the “Appointment Type Billing Behavior” column is either not set or set to "Non-Billable Encounter". You should check that column value and if it says “[EditValue is Null]” or "Non-Billable Encounter” then that means no Claim is created and no charge codes / CPT codes will be able to be added to the encounter.  This needs to be “Billable Encounter”. 

This value must be set when entering a new Appointment Type under Admin -> Lists -> List Item Entry OR from Scheduler -> Scheduler Options -> Appointment Types. Simply click in the cell and choose the correct selection from the drop down then click off the cell (or tab off) to save the selection.