How to calculate percent error for absolute zero?
It sounds like this is one of those canned labs that ask you to do standard things, even when they do not apply.

As you noticed, the percent-error formula needs you to use a value in the denominator.  If you use 0K in the denominator, you get a divide by zero.  If you use your 15K extrapolation, the percent error will always be 100%, no matter what the error actually was!

In such cases, I would look at the percent error with respect to your data.  Why?  Well, because somebody said you need to calculate a percent error and you might as well use a real datapoint rather than divide by 0 or some other silly result.  If you choose your coldest datapoint, you can say something about the percent error of the extrapolation process at the least.  Thus, if your coldest temperature recorded was 2C, you could say "My extrapolation suggests that absolute zero is 260C below my coldest recorded value.  The actual absolute zero is 275C below my coldest recorded value.  This gives me a 5.7% error on my measure of absolute zero relative to my coldest recorded value."