How to create a marginal effects plot for fixed effects regression with TSCS data?

Question

I am trying to replicate a marginal effects plot for the following model

model_7 <- plm(devexp_share_totexp2 ~ enop_seatslag + turnoutconslag + ix_turnoutenop_seatslag +
                 devexp_share_totexp2lag + growth_newlag + log_realinc_newlag + rightconslag + 
                 coalitionlag + post1991 + f1 + f2 + f3 + f4 + f5 + f6 + f7 + f8 + f9 + f10 + f11 + 
                 f12 + f13 + f14 + f15,
               data = state_data, index = c("state", "year"), model = "within", effect = "individual")

I'm trying to use the R package interplot to createa a marginal effects plot for the variables "turnooutconslag" and "enop_seatslag" but get the error

"Error in (function (classes, fdef, mtable)  : 
  unable to find an inherited method for function ‘sim’ for signature ‘"plm"’