Why can't covariate simulation runs be executed?

Dear everyone: I tried to execute the covariate simulation run, but an error always appeared: cov1 <- list(cov=cov.6, mean=list(wt=50), sd=list(wt=20), limits=list(wt=c(10,70)), fix=c("age","gender")) SIMrun(poppar=final.6,data="dataIV1.csv",model="modelIV2.txt",include=1:4,limits=NA,nsim=100,covariate=cov1) Error: poppar must be a PM_final object if covariate simulations are used. But if I did not add "covariate=cov1" into the command, the simulator run would be executed successfully. What is wrong with the covariate simulation run? Look forward to your comment! Kind regards!