sudo apt install flameshot

 $ sudo apt install flameshot
[sudo] password for dhwan:
Reading package lists... Done
Building dependency tree       
Reading state information... Done
The following package was automatically installed and is no longer required:
  libfprint-2-tod1
Use 'sudo apt autoremove' to remove it.
The following NEW packages will be installed:
  flameshot
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 288 kB of archives.
After this operation, 1.220 kB of additional disk space will be used.
Get:1 http://id.archive.ubuntu.com/ubuntu focal/universe amd64 flameshot amd64 0.6.0+git20191001-2 [288 kB]
Fetched 288 kB in 3s (103 kB/s)     
Selecting previously unselected package flameshot.
(Reading database ... 186313 files and directories currently installed.)
Preparing to unpack .../flameshot_0.6.0+git20191001-2_amd64.deb ...
Unpacking flameshot (0.6.0+git20191001-2) ...
Setting up flameshot (0.6.0+git20191001-2) ...
Processing triggers for desktop-file-utils (0.24-1ubuntu3) ...
Processing triggers for mime-support (3.64ubuntu1) ...
Processing triggers for hicolor-icon-theme (0.17-2) ...
Processing triggers for gnome-menus (3.36.0-1ubuntu1) ...
Processing triggers for man-db (2.9.1-1) ...
Processing triggers for bamfdaemon (0.5.3+18.04.20180207.2-0ubuntu2) ...
Rebuilding /usr/share/applications/bamf-2.index...