Case1: if you are on SQL Server 2017 CU3 or lower one, you have to separately install SQL agent package.

below is the steps to install the package

sudo yum install mssql-server-agent
sudo systemctl restart mssql-server

in case, if SQL agent is already installed, you can update the latest version as shown below.

Case2: if you are on SQL Server 2017 CU4 and higher one, then you don't have to separately install SQL agent package, you just need to enable SQL agent as shown below.

sudo /opt/mssql/bin/mssql-conf set sqlagent.enabled true
sudo systemctl restart mssql-server
sudo systemctl status mssql-server