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 (destination) dc2 (source) dc=contoso,dc=com (naming context)
It gets more fun when for example you are syncing a particular partition. In the following example again I am using dc1 and dc2 and specifying destination and source, but this time the source has to be expressed as the GUID of the Domain Controller. This can be found out by running another Repadmin command Repadmin /showreps against the source Domain Controller !.
Repadmin /showreps dc2 ( that will give you the guid of your source domain controller)
Then to sync a particular partition you would enter the following command.
Repadmin /sync cn=configuration,dc=contoso,dc=com dc1 (destination) (partners guid you got from previous showreps)
There is excellent help with Repadmin. Repadmin /help plus also Repadmin /experthelp. It is worth spending some time studying this to get it clear the way the syntax works and interacts.