This job could be using some prep_process call, but the execution was not possible on that Agent.
For example: SET &hnd# = PREP_PROCESS("&R3GETJOB_HOST#", "R3GETJOB.GET_JOBCOUNT", '*&R3GETJOB_JOBNAME#*', ...
I would suggest creating a test Job using the same Agent and Login configuration as the original Job and then the error should be clear to see (e.g. Host does not exists, or Access denied).
In your case, "could not be started on '<WIN>'", seems that you are passing the default Agent <WIN> as the host to be executed, so this will not work as this Host does not exist.