Drush 8 not working correctly on (LAMP) Ubuntu 16.04
Recently run into a similar problem on my local environment after an ubuntu update and php 7.2 became my default version.

make 7.1 or lower your default php version

I believe you should make php 7.1 the default or perhaps even a lower php version for Drupal 7 installation as not all modules are ready for php 7.2 (even some of Drupal 8 modules are not ready for php 7.2)

type this in your shell

$ sudo update-alternatives --config php


and this is what you will get 

There are 4 choices for the alternative php (providing /usr/bin/php).

  Selection    Path             Priority   Status
------------------------------------------------------------
* 0            /usr/bin/php7.2   72        auto mode
  1            /usr/bin/php5.6   56        manual mode
  2            /usr/bin/php7.0   70        manual mode
  3            /usr/bin/php7.1   71        manual mode
  4            /usr/bin/php7.2   72        manual mode
Press &lt;enter&gt; to keep the current choice[*], or type selection number:


Choose the appropriate version 

might need to update drush to a later version.

8.1.16 is out at this moment and might already have solved the problem you describe. Update using composer 

alternatively you can try to setup drush without composer

# get drush latest 8 version from 
# https://github.com/drush-ops/drush/releases
wget https://github.com/drush-ops/drush/releases/download/8.1.16/drush.phar

# make it executable
chmod +x drush.phar;

# move it to home directory or any other directory
mv drush.phar ~/drush;

# go to that directory
cd ~

# find out the path to the directory
pwd
# the output would look similar to this
# /home/your_user

# edit the following file 
nano .bashrc
# or 
# nano .bash_profile

# add this the end of the file
alias drush='/home/your_user/drush';

# use the alias you just setup
source ~/.bashrc;


might want to specify site directory when you invoke drush

as you drush might have a messed up setup/aliases

drush status -r /path/to/site/rootdir


doublecheck your settings.php

Does it include the correct database connections? Does the site actually run with those connection settings ? 

Look in your logs for any problems

in my system I can see apache errors like this 

tail -50 /var/log/apache2/error.log 


You might discover that some apache packages are missing or disabled just an example lets say mod_rewrite is not enabled

sudo a2enmod rewrite
sudo service apache2 restart


or you might find that some package is missing like mysql-client 

sudo apt-get install mysql-client