I'm trying to create a new theme by following
'http://devdocs.magento.com/…/front…/themes/theme-create.html' or
'Theming for Magento 2 - Starting from scratch'
this tutorial but once everything was done then I'm getting this annoying error: 
P.S: I have cleared all cache, reindex, deploy-static, maintenance:disable and things like that. At the moment I've installed Magento on a bitnami instance.
Does anyone have such an issue with his/her experience?
It may be permissions issue. In Bitnami documentation they suggest to do follow in case of 500 errors:
sudo find /opt/bitnami/apps/magento/htdocs/var -type d -exec chmod 755 {} \; sudo find /opt/bitnami/apps/magento/htdocs/var -type f -exec chmod 644 {} \; sudo chmod +x /opt/bitnami/apps/magento/htdocs/bin/magento-cli sudo chown -R daemon:daemon /opt/bitnami/apps/magento/htdocs/varCheck error log of your web-server. You'll get idea what's wrong.
– Wise Ltd. Mar 15 '18 at 23:13