It seems that periodically people encounter problems where the knit button is unresponsive:
Knit button not working (2018)
Knit button completely unresponsive (2019)
(I'm new so I can't post > 2 links, but google "knit to PDF not working R V4.0.0 RStudio V1.2.5042" to find third instance from 2020)
In these discussions, I don't see a lot of reference to R version and would like to add an observation: I have no issue knitting the following (default) document in R v4.1.0, but will generate a completely unresponsive blank screen in R v4.1.2:
---
title: "Untitled"
author: "Jordan Harshman"
date: "1/7/2022"
output: html_document
---
## RMarkdown
Anyone have any ideas as to why an update of base R could trip a problem with knitr? For now, the easy workaround is to use a previous version, I'm just posting this as additional info for what seems to be a repeating puzzle that may not have to do anything in the knitr package itself but something with base R.