Should I cook my pot roast on high or low?
Without any other information, it sounds like you have to assume that your slow cooker has only a warm setting (labeled "low") and a high setting, and use high for cooking. (Usually they have warm, low, and high.)

But it's possible that they're just being overly paranoid in the manual. I would personally try heating something on low (possibly just water, as a test), and seeing whether it boils or at least reaches and maintains a decent temperature.

The thing you're trying to avoid is the food spending too long in the "danger zone", below 140F (60C). If the low setting only just brings it above that, then it's going to take forever to heat up past it, and your food will be unsafe because all that time it wasn't hot enough. But on most slow cookers, the low setting brings it up to at least around 180F and on many, it even very slowly boils. If your heats that well, you can certainly use it for cooking. The one thing to keep in mind is that if it still takes a long time to come up to temperature, you may want to start it on high to get it all heated, and then switch to low for the rest of the cooking time.