Hallo
Shutter is a feature rich screenshot tool with a built-in "Edit" function that allows you to add lines, circles, numbers (they increase sequentially - very useful for showing a sequence of actions to be followed), etc. to screenshots.
You can install it via "Synaptic".
Unfortunately the Ubuntu 18.04 repositories seem to be missing the dependancies required to make the "Edit" function work.
Edit (24.02.2019)
A kind person has made a ppa available see:
You may prefer to use the ppa rather than the method given below.
End of edit.
The non-functional "Edit" option requires the "libgoo-canvas-perl" library which is not included in the Ubuntu 18.04 main archive. As a workaround, you can install the package and its dependencies from Ubuntu's 17.10 repository.
Proceed as follows:
A)
Download the "libgoocanvas-common" package:
https://launchpad.net/ubuntu/+archive/primary/+files/libgoocanvas-common_1.0.0-1_all.deb
B)
Download the "libgoocanvas3" package:
https://launchpad.net/ubuntu/+archive/primary/+files/libgoocanvas3_1.0.0-1_amd64.deb
C)
Download the "libgoo-canvas-perl" package:
https://launchpad.net/ubuntu/+archive/primary/+files/libgoo-canvas-perl_0.06-2ubuntu3_amd64.deb
-
Use GDebi to install the packages in the above order.
-
If Shutter is running "Quit" the application.
-
Restart Shutter - you should now be able to access the "Edit" function. If not you may have to re-boot your computer (this shouldn't be necessary for most users).
Source:
Many thanks to Ji m 
who posted this workaround here:
http://ubuntuhandbook.org/index.php/2018/04/fix-edit-option-disabled-shutter-ubuntu-18-04/
I've included the details, and not only a link here for the sake of redundancy (two copies are better than one).