I have encountered the same problem and I found the following solutionBefore going further, check this out
"These navigation elements are custom elements that only exist in the boost theme. The boost theme has it's own core renderer which is where they are defined. See theme/boost/classes/output/core_renderer.php. The functions are region_main_settings_menu() and context_header_settings_menu()."
Based on my understanding, the method region_main_settings_menu() can only be found in the boost core renderer. Hence, when trying to override the core renderer, instead of using
class theme_mytheme_core_renderer extends core_rendereryou have to use
class theme_mytheme_core_renderer extends theme_boost\output\core_rendererto point exactly to the boost core renderer.
Having done that, the error is gone.
Hope it helps.