If you have come across an issue of installing sql server on AWS Linux AMIs, especially when you are using one of those Free Tier Eligible AMIs, well its very likely that this is the solution for you. I came across this issue and got me stuck for a little while before I found the solution somewhere on the net. The problem is that the AMIs that you are using may not have the default repos in it.
Steps below.
sudo wget https://dev.mysql.com/get/mysql57-community-release-el7-11.noarch.rpm
sudo yum localinstall mysql57-community-release-el7-11.noarch.rpm
sudo yum install mysql-community-server
sudo systemctl start ms systemctl status mssql-server
Here is a video tutorial on how you can fix this.