As the error message says:

you should check the error log for details

Without details on what error is occurring and what context it’s occurring in (which is what is provided in the error log) it’s nearly impossible to know what the underlying cause is.

That being said, the first things I would check:

1.) File permissions (make sure the web files you uploaded are owned by www-data and not root.

2.) Double-check the config file you updated. If there is an option for a database table prefix make sure it matches what your database shows.

3.) MySQL does not listen publicly by default. You must change the address MySQL listens on in /etc/mysql/my.cnf to 0.0.0.0 for it to listen on all interfaces. Without this setting change no outside client can reach your database. Once you’ve made the change, restart the mysql service with service mysql restart