I get the request not found after full installation

I get the request not found after full installation. ‌I have the following files and directory in my foss dir:

root@debian:/var/www/html/foss# ls -l

total 19872

-rw-r--r-- 1 www-data www-data 8681 Apr 25 17:49 CHANGELOG.md

-rw-r--r-- 1 www-data www-data 2427 Apr 25 17:49 composer.json

-rw-r--r-- 1 www-data www-data 285829 Apr 25 17:49 composer.lock

-rw-r--r-- 1 www-data www-data 1406 May 25 13:50 config.php

-rw-r--r-- 1 www-data www-data 6008 Apr 25 17:49 config-sample.php

-rw-r--r-- 1 www-data www-data 1190 Apr 25 17:49 cron.php

drwxr-xr-x 6 www-data www-data 4096 Apr 25 17:49 data

-rw-r--r-- 1 www-data www-data 19874 Apr 25 17:49 di.php

-rw-r--r-- 1 www-data www-data 19904208 May 25 13:43 FOSSBilling.zip

-rw-r--r-- 1 www-data www-data 12017 Apr 25 17:49 foss-update.php

-rw-r--r-- 1 www-data www-data 1584 Apr 25 17:49 index.php

drwxr-xr-x 2 www-data www-data 4096 May 25 13:50 install

-rw-r--r-- 1 www-data www-data 1579 Apr 25 17:49 ipn.php

drwxr-xr-x 9 www-data www-data 4096 Apr 25 17:49 library

-rw-r--r-- 1 www-data www-data 11454 Apr 25 17:49 LICENSE

-rw-r--r-- 1 www-data www-data 16160 Apr 25 17:49 load.php

drwxr-xr-x 27 www-data www-data 4096 Apr 25 17:49 locale

drwxr-xr-x 45 www-data www-data 4096 Apr 25 17:49 modules

-rw-r--r-- 1 www-data www-data 11420 Apr 25 17:49 README.md

-rw-r--r-- 1 www-data www-data 469 Apr 25 17:49 rector.php

-rw-r--r-- 1 www-data www-data 1329 Apr 25 17:49 rewrite-missing.html

-rw-r--r-- 1 www-data www-data 615 Apr 25 17:49 robots.txt

drwxr-xr-x 4 www-data www-data 4096 Apr 25 17:49 themes

drwxr-xr-x 28 www-data www-data 4096 Apr 25 17:49 vendor

root@debian:/var/www/html/foss#

When i click on admin area, I get this error.