r - Post-hoc tests for repeated measures ANOVA with aligned rank transformed(ART) data

Question

I'm working on a 2x3 factorial design with two within-subject variables and I'd like to test for both main and interaction effects. Since the data is not normally distributed, I've performed an aligned rank transform on the data followed by a two-way repeated measures ANOVA (I'm using the "ARTool" package in R.) Here's how I've ran the code:

m <- art(Y ~ X1 * X2 + Error(Subjects/(X1 * X2)), data=myData)
anova(m)

The ANOVA shows a significant interaction effect between the factors X1 and X2. Consequently, I want to conduct a post-hoc test to find out the effect of X1 at each level of X2 and vice versa.

Currently, I'm struggling to find a way to do this. I've tried using the art.con() function as recommended by the authors:

art.con(m, "X1:X2")

However, when I run the code above I get the error 'Error in eval(predvars, data, env) : object 'X1' not found.'

On a side note, when I use the error term Error(Subjects) instead of Error(Subjects/(X1 * X2), I have no trouble getting post-hoc results using the art.con() function. However, changing the error term gives me different p-values, which I believe are the wrong results for my data.

Can anyone give me assistance on how to run post-hoc tests for aligned rank transformed ANOVA? Any help would be much appreciated.