you can ignore parsing errors - you won't see their effect ingame.
what is happening is that a mod yr installing is trying to reference something - usually a spell - that isn't present in the associated .IDS file. as a result, that particular script block won't do anything in-game (no other effects will be observed). the reason the identifier is missing in the .IDS file is either because the mod in question has an error or is referencing an old identifier; or some other mod you've previously installed has already altered the .IDS file in question. a fair few of the older mods - Kelsey, Tactics and Ascension all spring to mind - install their own, broken versions of some .IDS files. there was a good reason historically for this to happen; unfortunately, in the intervening years their authors haven't bothered to update them - so you often find these parsing errors if you've got these mods installed.
i think, if i'm not mistaken, that the new
BG2 Fixpack corrects the .IDS files and prevents the older mods overwriting them (altho this might be wish fulfillment on my behalf); in which case yr experiencing parsing errors for the first reason i specified: the mod yr installing is referencing an incorrect identifier. how's that for an answer?
the short answer, of course, is that you can ignore those errors: you will not see their effect ingame.