It's important that you just don't follow instructions blindly and also that you make decisions that are appropriate for you. If you're just a beginner choosing the default options and preferring (TeX) packages from the Ubuntu repositories over external sources is the best option.

Stop here and read the last paragraph again...

Really.

Cause

The instructions you probably followed suggested to not install symlinks, which is why all the resources, like binaries and manpages, are not in your environment (PATH variable) and Ubuntu doesn't know where these are installed or even if they are installed installed.

Also note the directories in the screenshot for a hint if you don't know where you installed TeX live (assuming that these aren't customized).

Correcting the mistake

As the instructions don't seem to provide how to setup the missing symlinks¹, reinstalling to the same location as before, but this time with creating symlinks option enabled, would be one option.

The other option would be to remove what TeX live installed (seems to be in /usr/local but /opt is also a possible location, only remove files and directories related to textlive, other programs may have stored files there too, more details see FHS) and install the version from the repositories. Upgrade to a newer LTS release if you need a newer version of TeX Live

¹ Possbily related: How to install “vanilla” TeXLive on Debian or Ubuntu?