Error: Class 'Drush\Sql\Sql' not found
Does your password contain a #? Try this test:

&lt;?php

function test_url($url) {
    $message = (parse_url($url) === false)?"parse_url unable to parse url: 
{$url}\n":print_r(parse_url($url),true);
    echo  $message;
}

test_url("mysql://user:password@site/database_name");
test_url("mysql://user:C0mPl3XP4ss@site/database_name");
test_url("mysql://user:Ci)(*$!@@site/database_name");
test_url("mysql://user:pass#word@site/database_name");
?&gt;


The last URL example fails to be parsed. This appears to be coming from 

vendor/drush/drush/src/Sql/SqlBase.php:dbSpecFromDbUrl().


I hit the same issue and traced it to a '#' in my MySQL user password. Changed the password to remove the '#' and everything is working.