tabset not working on windows

I have been using .tabset to organise my outputs. I moved to working on my laptop instead of desktop because currently I can't access my office (pandemic happening and all that). I tried to make a new code and the result was that instead of tabset I get:
#Results {.tabset} ##tab1
then some code
I also tried to run old code where tabset worked, but it also doesn't work now.
My laptop has been recently wiped clean, so it has latest version of R, Rstudio, Pandoc & rmarkdown.
The file structure is:

title: "Exp. 2.1a - Accuracy model and output"
output:
  html_document:
    df_print: paged
---
#Results {.tabset} 
##tab1

then, when I knit file to HTML, I get this message at the end, which I assume is the clue as to why it's not working. Any ideas on what this means? What am I missing?:

"C:/Users/Agata/AppData/Local/Pandoc/pandoc" +RTS -K512m -RTS Exp-2.1a--Accuracy-model-and-outputs.utf8.md --to html4 --from markdown+autolink_bare_uris+tex_math_single_backslash+smart --output Exp-2.1a--Accuracy-model-and-outputs.html --email-obfuscation none --self-contained --standalone --section-divs --template "C:\Users\Agata\Documents\R\win-library\3.6\rmarkdown\rmd\h\default.html" --no-highlight --variable highlightjs=1 --variable "theme:bootstrap" --include-in-header "C:\Users\Agata\AppData\Local\Temp\RtmpYJ5ft0\rmarkdown-str2618310523e5.html" --mathjax --variable "mathjax-url:https://mathjax.rstudio.com/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML" --lua-filter "C:/Users/Agata/Documents/R/win-library/3.6/rmarkdown/rmd/lua/pagebreak.lua" --lua-filter "C:/Users/Agata/Documents/R/win-library/3.6/rmarkdown/rmd/lua/latex-div.lua"