Strict Standards Error bootstrap navwalker

It means that the declaration of the start_lvl method in wp_bootstrap_navwalker should match the declaration of the method in Walker_Nav_Menu. It doesn’t.

function start_lvl( &$output, $depth ) {

VS.

function start_lvl( &$output, $depth = 0, $args = array() ) {

Make the arguments match exactly and you should be fine.

And you should probably not use the wp_ prefix as yours is not Core code.