Hi Mohammad,
Did you configure the database driver.See the AOLserver documentation
Even i got the same error.Try giving the following commands and then restart the AOLserver.
add database specific wrapper script and configure the postgres diver nspostgres
[root aolserver]# cd /usr/local/aolserver/bin
[root bin]# cp /var/tmp/openacs-5.2.0/packages/acs-core-docs/www/files/nsd-postgres.txt ./nsd-postgres
[root bin]# chmod 755 nsd-postgres