The result of this code
Full_data %>%
select(SF36HT_FOLLOW4) %>%
filter(!is.na(SF36HT_FOLLOW4))
is a data frame. But the mean function expects a vector of values. It looks like you are trying to do something like the following. The code below returns a data frame with one row and one column:
SF36_HT_M <- Full_data %>%
summarise(SF36HT_mean = mean(SF36HT_FOLLOW4, na.rm=TRUE))
Another possibility is to use the pull function to get a vector of values from a data column, which you can then run mean on directly. For example, the code below return a vector with one value (the mean of the "pulled" data):
SF36_HT_M <- Full_data %>%
pull(SF36HT_FOLLOW4) %>%
mean(., na.rm=TRUE)