Have you added any your account mods, or are you using the original default PHP-Nuke Your Account module? Is this a new install or nuke, or have you been up for a while with it working and you started getting this error?
That error is usually caused by not having sql tables in the database of mysql. So if you have modded it you need to add the tables supplied with the mod.
Just trying to sort out the variables here.
Martin