how to write italic script in Rstudio
The title is misleading as this error doesn't have anything to do with making the font italic. In Seurat v2, FeaturePlot does not return a ggplot2 object by default, so p in your case is NULL. You need to set do.return to TRUE in the FeaturePlot call.

You should instead do:

library(Seurat)

p &lt;- FeaturePlot(pbmc_small, head(pbmc_small@var.genes), do.return = TRUE)

for(i in 1:length(p)) {
  p[[i]] &lt;- p[[i]] + theme(plot.title = element_text(face = 'italic'))
}

cowplot::plot_grid(plotlist = p)