Follow these steps: 
1. Ensure the constituent's country is eligible for Address Accelerator:
  1. Select Configuration, International
  2. Select the appropriate country
  3. Mark the 'Addresses with this country are eligible for AddressAccelerator' checkbox
2. Ensure the user's security group has been given rights to edit addresses.
3. Ensure that AddressAccelerator has been unlocked for this database. For instructions, review How to install or update AddressAccelerator