Hi and welcome!
Do you maybe have NAs in your data? That would explain why you get no results for mean and median. If that is the case, you can just add na.rm=TRUE to your mean and median functions to get a proper result.
bmi %>% summarize(mean(HT, na.rm = TRUE), median(HT, na.rm = TRUE),mean(WT,na.rm = TRUE))
If that does not solve your problem, it would be good if you could post a reproducible example so that everyone can understand and look into your problem.