Sometimes the default portfolio archive page overrides the page you set to use /portfolio/ as the URL.
If this occurs, you will need to disable the portfolio archive page to be able to use the /portfolio/ URL.
You can do this by editing line 26 /wp-content/plugins/adios-addons/custom-posts/portfolio.php so it looks like the following:
'has_archive' => false,
After disabling the portfolio archive page, we still need to flush the url-rewrite rules.
This can be done in the Settings > Permalinks page in Wordpress Dashboard by changing the permalinks setting to something else, and then changing it back to its original value.