kayabeyumut very likely the full error message contains more text than that, or it doesn't come from Flarum. Can you copy the entire error? https://docs.flarum.org/troubleshoot
Are you saying you used the optional command-line installation wizard, or the default web installation wizard? Did you reach the end of the installation? Does a config.php
file exist in your Flarum folder?
If the wizard reached the end, you should be able to share your php flarum info
output with us, otherwise what's your PHP and MySQL version?