For those still sticking to the popular screenshot tool, Shutter is available to install in Ubuntu 19.04 via a third-party PPA.
Shutter is no longer available in Ubuntu Software since Ubuntu 18.10, because some old Gnome libraries required by the screenshot tool have been removed from Ubuntu universe repositories.
If you’re just looking for an alternative, I would recommend Flameshot which can be installed from Ubuntu Software.
1. Open terminal either via Ctrl+Alt+T keyboard shortcut, or from application launcher. When it opens, run command to add the PPA:
sudo add-apt-repository ppa:linuxuprising/shutter
Type user password (no asterisk feedback due to security reason) when it prompts and hit Enter to continue.
2. Then install Shutter either via Synaptic package manager or by running command in terminal:
sudo apt install shutter
Once installed, launch it from gnome software menu and enjoy!
To remove shutter, simply open terminal and run command:
sudo apt remove --auto-remove shutter
And remove the PPA via Software & Updates -> Other Software.