Your variables are inside your forest object, so you need to pick them there. This works for me.
forest<-structure(list(Author = c("Richard2012[1]", "Wang2015[2]"), coef = c(0.3, 0.45), low = c(0.1, 0.2), high = c(0.4, 0.55)), class = "data.frame", row.names = c(NA, -2L))
labeltext <- as.matrix(forest[,1])
library(forestplot)
#> Le chargement a nécessité le package : grid
#> Le chargement a nécessité le package : magrittr
#> Le chargement a nécessité le package : checkmate
forestplot(labeltext, mean = forest$coef, lower = forest$low, upper = forest$high)
Created on 2022-03-04 by the reprex package (v2.0.1)