Question:

The root cause is the wrong data type.

The factor type has no max method

#create a vector of class vector
factor_vector <- as.factor(c(1, 7, 12, 14, 15))

#attempt to find max value in the vector
max(factor_vector)

#Error in Summary.factor(1:5, na.rm = FALSE) : 
#  'max' not meaningful for factors

Solution:

Convert to numeric value or string, here convert to numeric value.

mydata$value<-as.numeric(mydata$value)
is.numeric(mydata$value)

#convert factor vector to numeric vector and find the max value
new_vector <- as.numeric(as.character(factor_vector))
max(new_vector)

#[1] 15

Full error:

#create a vector of class vector
factor_vector <- as.factor(c(1, 7, 12, 14, 15))

#attempt to find max value in the vector
max(factor_vector)

#Error in Summary.factor(1:5, na.rm = FALSE) : 
#  'max' not meaningful for factors

Other (numeric value, string, date type can be the maximum value)

Numeric value, string and date type can all be the maximum value, and similarly, the minimum value can be obtained.

numeric_vector <- c(1, 2, 12, 14)
max(numeric_vector)

#[1] 14

character_vector <- c("a", "b", "f")
max(character_vector)

#[1] "f"

date_vector <- as.Date(c("2019-01-01", "2019-03-05", "2019-03-04"))
max(date_vector)

#[1] "2019-03-05"