After installing the demo content is comes a message:
Error: An error has occurred.
500 Unknown column 'a.note' in 'field list
The 500 error is due to the fact that the demo content is not for the joomla version you are using. In every template info is written for which joomla version is done the demo sql file or quickstart. With quickstart there will not be any issues. If you have installed the sql demo file in latest joomla version, it is normal to have that message because the database is not the same and is needed to update to current joomla.
To fix this error is needed to Fix Your Database Schema.
After a couple of seconds, you will see the "Database table structure is up to date" message:
By finishing this process the error will disappear from your joomla website.