Can I cook my apple butter on high when it suggests low?
While different cookers have different opinions about how hot "high" and "low" actually are, I still wouldn't gloss over such a large discrepancy, unless I knew enough to make the call. 

However, you can easily lower the temp of your crockpot on it's high setting. Try putting a terrycloth rag between the metal case and ceramic pot. This will reduce the heat transfer from the coils to the food, resulting in a stable but lower temp. Use a thermometer to check before/after temps and adjust the amount of "insulation" to get the temp you want. Low on my cheap consumer crockpots is around 120-140F.