Erythritol is an artificial sweetener. It's a sugar alcohol like maltitol and lactitol, but is generally agreed to be much lower-GI and much easier on the stomach. Many people who have problems with other SAs find that they can tolerate erythritol better and that they can completely deduct all the carbs.
This article is a good general overview of SAs, and
this article discusses erythritol in particular a little bit more.
Erythritol by itself is about 70% as sweet as sugar, and unlike Splenda, does not dissolve completely as soon as it hits liquid. It also caramelizes and has some of the other baking properties that Splenda lacks, so it's very popular with serious LC bakers. By itself, it has a vaguely minty aftertaste, often referred to as the "cooling effect" -- it doesn't taste minty so much as it makes your mouth feel cool like peppermint does. When you combine erythritol and Splenda, the flavor is better and you can use less total sweetener than if you use either one alone -- for example, I replaced 1 1/4 c Splenda with 1/4 c erythritol + 1/2 c Splenda in my banana bread recipe, and it improved both taste and texture. I also find the combo of unsweetened chocolate + Splenda to have a slight bitter aftertaste, but adding in erythritol seems to fix the problem.
To answer your question, no, you can't just leave it out of the recipe. You could try subbing with all Splenda, but be aware that this may negatively affect the flavor and texture. It will also result in a higher carb count, because most people agree that carbs from erythritol are pretty much 100% deductible, and the substitution will add an additional 12g to the total for the recipe. You can order erythritol online from pretty much any LC product store such as
Netrition.com, and some health food/LC stores carry it as well. I'm inferring from your question that you're relatively new to LC baking, and that you will probably have to hunt down some of the other ingredients as well, such as liquid Splenda and vital wheat gluten, so ordering it online might be the best choice.
Alternatively, you could try subbing in another SA-based artificial sweetener such as DiabetiSweet, which my Wal-Mart carries (in the pharmacy section with the glucose test strips and diabetic stuff). The amount used in the recipe probably won't cause you significant distress or net-carb problems unless you are very sensitive. There is a discussion of DiabetiSweet in that second article I referenced above -- just scroll down the page a bit.