How to show output in console when writing an RMarkdown notebook?

From the official website (emphasis by me):

By default, RStudio enables inline output (notebook mode) on all R Markdown documents, so you can interact with any R Markdown document as though it were a notebook. If you have a document with which you prefer to use the traditional console method of interaction, you can disable notebook mode by clicking the gear in the editor toolbar and choosing Chunk Output in Console.

This sets the chunk_output_type option in the YAML front-matter of your R Markdown document to console:

---
editor_options:
  chunk_output_type: console
---

Adding those lines to the YAML front-matter without clicking any GUI option will of course lead to the same result.

Under Options in Tools, choose the options for R Markdown, change the tick for "Show output inline...." to "untick". Good luck!