How to make standalone package available by my LaTeX installation?

I want to use LaTeX notes in Xournal++ on Ubuntu to annotate on .pdf.
However when I click on insert a LaTeX formula, I got:

Could not find LaTeX package ‘standalone’. Please install standalone (found in texlive-latex-extra) and make sure it’s accessible by your LaTeX installation).

I went online to search for help and tried:

sudo apt-get install texlive-latex-extra

which gave:

texlive-latex-extra is already the newest version (2019.202000218-1).
0 upgraded, 0 newly installed, 0 to remove and 211 not upgraded.

I tried a tlmgr install with:

$ tlmgr install standalone
(running on Debian, switching to user
mode!)

/usr/bin/tlmgr: unexpected return value from verify_checksum: -5

I finally went here because of the unexpected return value:

tlmgr –verify-repo=none install standalone (running on Debian,
switching to user mode!)

tlmgr: Local TeX Live (2019) is older than remote repository (2020).
Cross release updates are only supported with
update-tlmgr-latest(.sh/.exe) –update See
https://tug.org/texlive/upgrade.html for details.

Are there other steps I am missing or am I doing it completely wrongly?

sudo add-apt-repository ppa:apandada1/xournalpp-stable
sudo apt update
sudo apt install xournalpp