Sorry to necro, but I think there isn’t a straight answer to the OP…reinstalling isn’t always an option.
If you are using php7+ and php-fpm (on a raspberry pi stretch), the fpm service needs to be manually restarted after adding apcu. ie sudo systemctl restart php7.0-fpm.service…simply restarting the apache server does not do this automatically for you (same for making changes to fpm/php.ini). This may be worth mentioning in the documentation relating to the errors that tend to come up on a new NC install:
- “your php opcache settings are not right” and 2) “no memcaching set up”.
Hopefully this helps someone googling for a similar problem.