Causes fatal error on activation

Get this error on activation in WordPress 6.3.1:

Fatal error: Uncaught TypeError: call_user_func_array(): Argument #1 ($callback) must be a valid callback, non-static method MSAB::plugin_activation() cannot be called statically in XXX/wp-includes/class-wp-hook.php:310 Stack trace: #0 XXX/wp-includes/class-wp-hook.php(334): WP_Hook->apply_filters(”, Array) #1 XXX/wp-includes/plugin.php(517): WP_Hook->do_action(Array) #2 XXX/wp-admin/plugins.php(194): do_action(‘activate_mobilo…’) #3 {main} thrown in?XXX/wp-includes/class-wp-hook.php?on line?310