First make sure you have openssl installed. You will need version 1.0 since Astrill does not support 1.1. To install
sudo apt install openssl=1.0.2g-1ubuntu4.8
Then install shared libraries, first check which version you want by running
apt-cache search libssl | grep SSL. Example output:
libssl-dev - SSL development libraries, header files and documentation
libssl-doc - SSL development documentation documentation
libssl1.0.0 - SSL shared libraries <-- version number
Install the library with the correct version:
sudo apt-get install libssl1.0.0
From the conversation in the comments, you also need to install
libssl-dev, again, make sure the version is 1.0.
sudo apt install libssl-dev=1.0.2g-1ubuntu4.8
It is possible you need to install the packet
ca-certificates as well.
If the above did not solve the problem, open the file
sudo and add a new line at the end
Dependin on yuor system, the path could be different. Two other locations to try are
/usr/lib/openssl-1.0, see here and here.