Flameshot screenshot manager is not included in standard Devuan repositories, so we need to add Stretch-Backports repository (from Debian) in order to install it on your Devuan Linux system.
Adding Stretch-Backports repository
Open file /etc/apt/sources.list .
$ sudo vim /etc/apt/sources.list
In my case:
and add this line:
deb http://ftp.debian.org/debian stretch-backports main