I have fixed that error and it was wrong php version was showing on phpinfo.
I have managed to get to the installation page and selected the database, admin, site name etc, but now again 500 page error afater installation went OK.
Log error
PHP Fatal error: Uncaught UnexpectedValueException: The stream or file "/var/www/html/forum6/public/../storage/logs/flarum-2019-03-20.log" could not be opened: failed to open stream: Permission denied in /var/www/html/forum6/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php:107\nStack trace:\n#0 /var/www/html/forum6/vendor/monolog/monolog/src/Monolog/Handler/RotatingFileHandler.php(119): Monolog\Handler\StreamHandler->write(Array)\n#1 /var/www/html/forum6/vendor/monolog/monolog/src/Monolog/Handler/AbstractProcessingHandler.php(39): Monolog\Handler\RotatingFileHandler->write(Array)\n#2 /var/www/html/forum6/vendor/monolog/monolog/src/Monolog/Logger.php(344): Monolog\Handler\AbstractProcessingHandler->handle(Array)\n#3 /var/www/html/forum6/vendor/monolog/monolog/src/Monolog/Logger.php(707): Monolog\Logger->addRecord(400, Object(Error), Array)\n#4 /var/www/html/forum6/vendor/flarum/core/src/Http/Middleware/HandleErrorsWithView.php(87): Monolog\Logger->error(Object(Error))\n#5 /var/www/html/forum6/vendor/flarum/core/src/Http/M in /var/www/html/forum6/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php on line 107
