I have found that
SSerror = SStotal - SStreatment
which makes sense to me alongside an example of a one-way ANOVA, however, how does this apply to a factorial ANOVA?
If you have equal n and a full factorial, you can subtract all the effect sums of squares from sum of squares total. If you have unequal n, you can compute the sum of squared deviations of values from their mean in each cell and then summing. Or, simply, analyze it as a one-way to get SSE.