1 - Check the MySQL and servers error log

2 - Check the read+write permissions on the directory/file where Elgg will install

I can give more details about my server and maybe it will help solve the problem
I have a VM private server that has ContOs7 installed on it and this system has free web hosting of the type https://kloxong.org/
The domain is a full domain and not a sub subdomain or subdomain folder.
All files are in 777 permissions including DATA folder (I called it by another name and outside of installation folder)
Please note that I am now getting an HTTP ERROR 500 server error

I enclose an excerpt from a log I found on the server (I changed the identifying details to a mark **)

--

2021/01/08 12:13:41 [error] 32451#32451: *1742 FastCGI sent in stderr: "PHP message: [2021-01-08 12:13:41] ELGG.WARNING: Deprecated in 3.3: 'simplecache_lastupdate' config option is deprecated. Use 'lastcache'  {"backtrace":["[#9] /home/admin/z****/vendor/elgg/elgg/engine/classes/Elgg/Config.php:409","[#8] /home/admin/z****/vendor/elgg/elgg/engine/classes/Elgg/BootService.php:103","[#7] /home/admin/z****/vendor/elgg/elgg/engine/classes/Elgg/Application/BootHandler.php:99","[#6] /home/admin/z*****/vendor/elgg/elgg/engine/classes/Elgg/Application/BootHandler.php:47","[#5] /home/admin/z****/vendor/elgg/elgg/engine/classes/Elgg/Application.php:215","[#4] /home/admin/z*****/vendor/elgg/elgg/engine/classes/ElggInstaller.php:872","[#3] /home/admin/z****/vendor/elgg/elgg/engine/classes/ElggInstaller.php:92","[#2] /home/admin/z****/vendor/elgg/elgg/engine/classes/Elgg/Application.php:545","[#1] /home/admin/z****/install.php:11"],"process_id":5623,"memory_peak_usage":"8 MB","memory_usage":"8 MB","url":"/install.php?step=settings","ip":"**.***.***.***","http_met" while reading response header from upstream, client: 94.159.137.239, server: zvinisim.ml, request: "GET /install.php?step=settings HTTP/1.1", upstream: "fastcgi://unix:/opt/configs/php-fpm/sock/php73m-admin.sock:", host: "******.ml"

--

And also the RewriteBase section from the .htaccess file

-- #RewriteBase /sites/elgg/
RewriteBase  http://******.ml
#
# here, only without the # in front.
#
# If you're not running Elgg in a subdirectory on your site, but still getting lots
# of 404 errors beyond the front page, you could instead try:
#
RewriteBase /

--

Try this tutorial.

1. Let's say your Elgg root installed directory is:

/var/www/elgg/www/

And data directory is:

/var/www/elgg/data/

Then your Elgg config is located at:

/var/www/elgg/www/elgg-config/settings.php

If you can't find this file then copy file from:

/var/www/elgg/www/vendor/elgg/elgg/elgg-config/settings.example.php

And paste + rename it on:

/var/www/elgg/www/elgg-config/settings.php

2. Open your Elgg config file and fill out these params (I did it with some examples):

$CONFIG->dataroot = "/var/www/elgg/data/";
$CONFIG->wwwroot = "/var/www/elgg/www/";
$CONFIG->dbuser = 'root';
$CONFIG->dbpass = '12345';
$CONFIG->dbname = 'elgg';
$CONFIG->dbhost = 'localhost';
$CONFIG->dbport = '3306';
$CONFIG->dbprefix = 'elgg_';

Of course, you should create a database 'elgg' with utf8mb4 encoding.

I recommend to delete the existing Elgg database before.

Note: You can learn more reading the comments in the mentioned file.

3. Go to /var/www/elgg/data/ and delete all folders/files.

4. Find out your webserver username.

Usually, www-data or apache or httpd.

Note: Learn this tutorial to find this name.

5. Let's say your webserver name is:

www-data

Now use these commands via console:

chown -R www-data:www-data /var/www/elgg/data/

chmod -R 750  /var/www/elgg/data/

chown -R www-data:www-data /var/www/elgg/www/elgg-config/

Done.

Now you've a manual pre-setuped Elgg and can try to install it again skipping some steps.

If you've errors then check your server logs again.

Note: there're already discussed topics about CentOS fails - please read them before start using this tutorial.

Good luck!