The problem with your code is in the line:
for(i in length(namevector))
You need to ask yourself: what is length(namevector)? It's one number. So essentially you're saying:
for(i in 11)
df[,i] <- NA
Or more simply:
df[,11] <- NA
That's why you're getting an error. What you want is:
for(i in namevector)
df[,i] <- NA
Or more simply:
df[,namevector] <- NA