tlmgr: Remote repository is newer than local (2017 < 2018)

This seems to do the trick. The solution is to downgrade the repository to 2017 version. This seems to be easy enough, but a quick search for the error did not suggest anything of this sort.

sudo tlmgr option repository ftp://tug.org/historic/systems/texlive/2017/tlnet-final

You don't have to upgrade at all, you can stop at tl2017 (lots of people installed tex years ago and have never updated)

However if you want to update from one texlive to the next without doing a full install (which is the recommended route) then the texlive team have a page detailing what you need to do

https://www.tug.org/texlive/upgrade.html

Note that this URL is exactly the address given in the error message that you quoted in the question.

TL; DR;

Get a fresh install of TeX Live, though you said you don't want to. The installation is easy and the newer one would be distinguishable from the older ones.

Story

I've been having texlive-latex-base by using apt-get on Ubuntu 18.04.
sudo apt-get install texlive-latex-base

When I was trying converting a Markdown file to a pdf file using pandoc I originally ran into a problem Error in TeX Live – Font … not loadable: Metric (TFM) file not found, then here.

But, immediately, I had a feeling that if I take one of these prescriptions I would be going through more uncertain problems in the later steps by intuition.

So, I decided to install an up-to-date version of TeX Live.
Starting from there, please follow the directions on the web page and it will take some time, maybe hours depending on your network condition.