I cannot find the plugin to simply add css and java script

Hi, I just signed up.

Installed/activated the WPMU DEV Dashboard plugin (at maximyou.com), but unable to finish set up, getting "We could not find your new website" messages.

At your plugin directory, looking for 'Simple Custom CSS and JS' plugin equivalent - in reference to https://premium.wpmudev.org/blog/twenty-seventeen-wordpress-theme/ and https://premium.wpmudev.org/blog/theme-modification-plugin/ so far unsuccessfully. Does it exist or must I buy separately?