SEIR model not working

Hi, It's much easier for us to help if you provide your example as a reproducible example, called a reprex .

After adding the missing *s to the differential equations (see below) it ran without errors for me, with all 4 lines plotting.

What version of dSolve are you running?

SEIR.dyn <- function(t, var, par) {
  #variables and parameters
  S <- var[1]
  E <- var[2]
  I <- var[3]
  R <- var[4]
  N <- S + E + I + R

  beta <- par[1]
  gamma <- par[2]
  delta <- par[3]
  #differential equations
  dS <- (-beta*S*I)
  dE <- (beta*S*I) - (delta*E)
  dI <- (delta*E) - (gamma*I)
  dR <- (gamma*I)
  list(c(dS, dE, dI, dR))