More than likely what could be happening is that you are trying to create a field mapping that does not match what you have setup in the ''Rules'' section. For example if in my rules for the link I have it set to create and update the entities on both sides ''bidirectional'' I MUST have the field setup as ALM <-> TFS if the field is a required field in both endpoints. This also works the other way I cannot setup a field as ALM <-> TFS if my rules are setup to only create and update on one endpoint. So initially you will need to decide if you want the link to go in one direction or in both (bidirectional). Once you decide that and set your rules up then you will map the fields accordingly.
If you setup the link as ''bidirectional'' you can still have some fields mapped as ''unidirectional'' but you could only do that on fields that are only ''required'' on one endpoint. However, if you setup the link as ''unidirectional'' you cannot create mapped fields that are ''bidirectional.''
Hopefully this makes sense; you can also take a look at the sync user guide below which goes into more detail.
What I describe above applies to synchronizing between any two types of endpoints be it TFS, clearquest, Jira, ALM, etc...