Sorry for the slow reply, your post somehow went through the cracks. You need to set
factorize = FALSE in the call to use numerical covariates.
For the example data that would be:
data(obk.long, package = "afex")
aov_ez("id", "value", obk.long, between = c("treatment", "gender"),
within = c("phase", "hour"), covariate = "age",
observed = c("gender", "age"), factorize = FALSE)
In your case this would be:
fit.grat2 <-aov_car(grat ~ tgrat+condition*time + Error(PPT/time),
data=mf_long2, covariate=centgrat, factorize = FALSE)
I should probably add a corresponding note to the documentation.