What MODX version did you upgrade from?
There's a good chance that running this query in PhpMyAdmin (after selecting you MODX DB) will solve your immediate problem:
ALTER TABLE `modx_site_content` ADD COLUMN `alias_visible` INT( 2 ) NOT NULL DEFAULT '1';
If you upgraded directly from a very old version of MODX, though, you may be missing other DB fields as well.
In that case, if you have a backup from before the upgrade, I'd suggest reverting to it, then upgrading in stages -- installing every x.x.0 version newer than your backup version (e.g., 2.2.0, 2.3.0, 2.4.0, 2.5.0, 2.6.0, 2.7.0, 2.7.1).
