After I updated prestshop 8.0.3 to version 8.1.0 the option to set the master page doesn’t work anymore, it’s just any master page, probably a theme template Warehouse 4.6.1, not possible to change.
You can contact the item author but it may be that the theme is not ready for 8.1.0 version.
You can restore the last working backup or downgrade the software to the previous state till you find a solution