t-1 has tmax rows but n[1,] has tmax+1 rows, so in this case 10 and 11 rows respectively, that is why you are getting that error message, if you match both lengths the plot works.

tmax <- 10
t <- 1:tmax
n <- matrix(0, nr = 5, ncol = tmax + 1)
n[,1] <- c(10, 10, 10, 10, 10)
plot(t-1, head(n[1,], tmax), type='l', xlab="t", ylab="N")

^{Created on 2019-11-20 by the reprex package (v0.3.0.9000)}