Your register_navwalker function is loading navwalker with this line:
require_once get_template_directory() . './class-wp-bootstrap-navwalker.php';
where you concatenate the path of the theme with this string:
'./class-wp-bootstrap-navwalker.php'
and the dot there at the beginning is a problem. The result would look like this:
DOMAIN/wp-content/themes/THEME./class-wp-bootstrap-navwalker.php
remove the dot and it should find and load the script.
EDIT:
Also you shouldn’t change your original post or question, but add to it. Otherwise comments and answers might not make sense anymore.