Repadmin is a command line tool that is of course extremely useful when it comes to troubleshooting replication problems. However as with all command lines the syntax has to be exactly right otherwise you will get a negative experience. The thing to remember about Repadmin is that you often have to conform to certain naming conventions to get a successful result from the input. For example, If you wanted to replicate between two Domain controllers dc1 and dc2. You have to specify the destination and the source and the naming context. e.g.

Repadmin /replicate dc1