I got it to work on nextcloud 13.0.4 debian 9 with apache on php 7.2.
Don forget if you use Let’s encrypt the active config file is nextcloud-le-ssl.conf instead of nextcloud.conf.
Like ludovicis in another thread said (and the pico administrator settings suggest) you have to add before the start of <directory /your/files/>:
RewriteEngine On
RewriteRule /sites/(.*) https://yourdomain.com/index.php/apps/cms_pico/pico/$1 [QSA,L]
And to activate the changes:
(sudo) systemct restart apache2