This error cam from file app/config/parameters.php
In Prestashop 8.x, 2 new attributes have been added : "api_public_key" and "api_private_key".
In my case, I rolled back to the previous file (to have old batabase names) and error comes because those 2 attributes were missing. I got them in my history so I could let them back and it's ok.
When upgrading 1.7 to 8, always manually backup your files and your bdd to replay upgrade if it is necessary (and it was for me !)
Hope it helps for futur upgraders.