It seems that you've cloned the server agent repository which doesn't contain any binaries, only the source code.
You should rather download installation package and unpack it into your preferred location, this way you will be able to start the server agent process by running
If you want to build it from sources you can execute
maven package command and it will produce
ServerAgent-x.x.x.zip file under "target" folder which you will be able to unpack where required and run the
startAgent.bat script from there