This is telling you there is another port in STP that is forwarding and this one is down because enabling it would create a layer 2 loop. This is the protocol working in the background not a configuration state you can change directly.
You will need to identify all the layer 2 connections for STP in the topology and figure out if the automatic selection by these default criteria is what you want or not. If not, then adjustment of parameters in STP will be needed to shift the forwarding port status to the desired link.
Steve Puluka BSEET - Juniper Ambassador IP Architect - DQE Communications Pittsburgh, PA (Metro Ethernet & ISP) http://puluka.com/home