Step 4a - add stringr to package library

[mollytest24]

At the section "## Summary statistics for sample groupings", function str_remove requires stringr package.

Add the package stringr to "if (!require("pacman")) install.packages("pacman")
pacman::p_load(knitr, here, tidyr, dplyr, vegan, glue, scales, datawizard)" in first code chunk setting up packages.