Add SQL Server Agent to existing SQL Installation
Try to run a repair intallation from the installation media and then install service pack and cu after that.

You can run the repair from the command prompt:

Setup.exe /q /ACTION=Repair /INSTANCENAME=instancename  


Or by opening the installation center and select Maintenance in the left-hand navigation area, and then click Repair to start the repair.

Step-by-step instructions are found in the documentation