why I get "NA" as a result of min() and max() in my dataframe?

I have a dataframe(mydat) that has no any missing data.

I use below commands:

min(mydat)

max(mydat)

for achieving the minimum and maximum values of my dataframe but for both the above functions, I get

NA

as a result in the console.

Now, I really confused and I don't know why I get that result. I appreciate if anybody shares his/her comment with me.