How can I assign a block to a panel or mini-panel?
Mini panels are blocks and you can add Mini Panels to Panels. You can't add blocks themselves to Panels, but usually most of the content that you can display in a block, you can actually display in Panels, content such as Activities, Menus, Mini panels themselves, Miscellaneous, Pages elements, Views, Widgets, Nodes, Custom content and more.

See Drupal - Blocks vs Context vs Panels.