Add Smart Lock screen security options to Amazon Fire HD 8?
Only a few devices support Smart Lock(mainly Motorola and Samsung)

If you are having trouble finding Smart Lock, try


Go to Settings > Security > Screen lock > Set the Pin Lock
After setting the Pin Lock go back to the Security Menu, at the Bottom of 
security menu you will find Trust Agent
Go to Settings > Security > Trust Agent
Enable Smart lock ( Google )
Select Smart Lock