Have you created the database to be used for Elgg? You need to do that on your own. And if the database exists, are you sure that the database name, database user and password you have entered are correct? These must match with the database name and the database user must have the permission to access the database. The error message indicates that there's a problem with connecting to the database. But which of the possible reasons I listed for the connection problem I can't tell from the error message as it could be either of them.

I am facing the same issue while installing :

"An unrecoverable error has occurred and has been logged. If you are the site administrator check your settings file, otherwise contact the site administrator with the following information:  "

I am using ELGG 2.3.5, with MySQL installed in my Windows 10 system & rest software in WSL (apache, php)

I could access db from WSL bash.

root@SAVYAS-LAP:/var/www# mysql -u root -p --host=127.0.0.1 --port=3306 --protocol=TCP
Enter password:
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 37
Server version: 5.7.21-log MySQL Community Server (GPL)

..

..

mysql>

I checked that I have settings.py correct 

root@SAVYAS-LAP:/var/www# grep db elgg-config/settings.php
 * @global string $CONFIG->dbuser
$CONFIG->dbuser = 'admin';
 * @global string $CONFIG->dbpass
$CONFIG->dbpass = <<passwd>>
 * @global string $CONFIG->dbname
$CONFIG->dbname = <<db-name>>
 * @global string $CONFIG->dbhost
$CONFIG->dbhost = '127.0.0.1';
 * @global string $CONFIG->dbprefix
$CONFIG->dbprefix = '';
//$CONFIG->db['split'] = true;
//$CONFIG->db['write']['dbuser'] = "";
//$CONFIG->db['write']['dbpass'] = "";
//$CONFIG->db['write']['dbname'] = "";
//$CONFIG->db['write']['dbhost'] = "";
//$CONFIG->db['read'][0]['dbuser'] = "";
//$CONFIG->db['read'][0]['dbpass'] = "";
//$CONFIG->db['read'][0]['dbname'] = "";
//$CONFIG->db['read'][0]['dbhost'] = "";
//$CONFIG->db['read'][1]['dbuser'] = "";
//$CONFIG->db['read'][1]['dbpass'] = "";
//$CONFIG->db['read'][1]['dbname'] = "";
//$CONFIG->db['read'][1]['dbhost'] = "";
 * @global bool $CONFIG->db_disable_query_cache
$CONFIG->db_disable_query_cache = false;

(sanitizing passwd & dbname )

Any help / pointers  ... please ??

1. Check your server logs
2. Try to edit this:

//$CONFIG->dbhost = '127.0.0.1';

$CONFIG->dbhost = 'localhost';

You can try their support forum. Better: make a PHP file and call phpinfo() and look for the error location there. Delete the phpinfo file when you're done.

Most probably, you are using some bad written plugins. I also got fatal error when I tried to activate some plugins.
This may be a reason. If you are using any third party plugin, try to deactivate it.
In hypewall, there is a error of undefined function, because of which, I got fatal error.
It may be in contradict of other plugins.
In hype maps open, there is also error, because of which it is not possible to run upgrade.php
If you are using third party plugins, try to remove it.

If the problem occurs already during installation, the error could be due to a problem connecting with the database (softaculous might causing a problem). Without knowing the error log entry it's difficult to help. It could also be a problem with an older Elgg version getting installed but already php 7.0 used on the server because Elgg is compatible with php 7 only since version 2.3.

Free hosting is most likely never a good idea for Elgg. Elgg is not like a simply CMS or forum framework but has slightly higher hardware requirements. So, you might not get happy with it on a free hosting plans (not even taking into account that a free hosting plan might not even provide the necessary config requirements to be able to install Elgg at all).

My suggestion: try installing Elgg not with softaculous. Also, ask the support of the hoster about the error log (Apache and php error log) and if mod_rewrite is supported and if you can create a directory outside the document root folder not getting blocked by open_basedir. If any of these requirements is not fulfilled, the hosting plan won't work for Elgg.

For testing Elgg you can also install it locally (if Linux is available no problem or if on Windows with XAMPP). Then you can find out if Elgg is useful for you and if the answer is yes look for a suitable server / hoster (most likely not free hosting but it must not be too expensive either).

The error message itself doesn't tell you what error occured but only when the error occured. The number is the unix timestamp of the time of the error. Check the server log (Apache and/or PHP error log). There will be an entry at the time of the error (and the entry will also contain the same timestamp number). This error log entry will provide more information about what's wrong. Without knowing more we can't give you any better advice.

Or if you have made any changes on your site or server just before the error occured (updating Elgg or a plugin or adding a new plugin or modifying any file) this would also give some indication of what might be wrong.