How can I perform a TukeyHSD with continuous and categorical variables?

I am trying to perform a TukeyHSD post-hoc test on my data. I have some variables that are categorical and one variable that is a numerical continuos variable (it represents different ranges of altitude, so the 1 refers to the range 1000-1100, the 2 refers to 1101-1200 and so on).

When I try to perform the Tukey test I get an error.

The code I used is the following: TukeyHSD(model1, "var1:var2:var3:var4:var5", conf.level = .95)

where var5 is the continuous one.

The error I get is the following: In qtukey(conf.level, length(means), x$df.residual) : NaNs produced

What could be the problem?

Thank you

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