How to install annotator from elementary OS appcenter in Ubuntu?

There are several ways to install Annotator.

PPA

You can use the PPA maintained by Panda Jim (who also happens to maintain the GIMP PPA, among many others). Enter the following commands one by one

sudo add-apt-repository ppa:ubuntuhandbook1/annotator
sudo apt update
sudo apt install com.github.phase1geo.annotator

The PPA supports Ubuntu 20.04+.

Flatpak

You can use flatpak to download it from the Elementary appcenter repositories.

flatpak install https://flatpak.elementary.io/repo/appstream/com.github.phase1geo.annotator.flatpakref

Manually install from source

First install the dependencies.

sudo apt install meson valac debhelper gobject-2.0 glib-2.0 libgee-0.8-dev libgranite-dev libxml2-dev libgtk-3-dev libhandy-1-dev

(libhandy for Ubuntu 20.04 is available in this PPA. In later versions of Ubuntu, it is available in the official repositories.)

Then download the source and run the install script.

git clone https://github.com/phase1geo/Annotator.git --depth=1
cd Annotator
./app install

Flatpak (manual install)

Download the source, and manually build and install with flatpak (the software is not yet available on flathub).

git clone https://github.com/phase1geo/Annotator.git --depth=1
cd Annotator
./app flatpak

And you can open the app from menu (or run it with flatpak run com.github.phase1geo.annotator).