How can I install TLP in Ubuntu 18.04?

You are reading old instructions off the internet. tlp is included in Ubuntu's universe repository so all you need to type is:

sudo apt install tlp

If you want to utilize the Radio Wizard also use:

sudo apt install tlp-rdw

Thinkpad users can install optional packages for battery threshold levels:

sudo apt-get install tp-smapi-dkms acpi-call-dkms

Source: Welcome to TLP!

For a person who is more into the UI, do the following.

$ sudo add-apt-repository ppa:linuxuprising/apps
$ sudo apt-get update
$ sudo apt-get install tlpui

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