I have a 2010 Fxstc, while riding the tail light flickers.A friend riding behind me said the brake light is working.It comes on and off but tail light flickering just riding down the road.I checked bulb it looks good.

 

You likely have a bare spot in wiring insulation or a break in the wire beginning somewhere on that circuit. Usually not fun tracing these things down.
Just tracked down and fixed a couple of lighting wire issues on wife's Deluxe. Fortunately this time around they were relatively easy to spot and repair.

 

Swap the left and right bulbs and see if it still flickers on the LH side. If not, it's the bulb. If so, start the troubleshooting process.

 

Start with the easy thing first
Replace the bulb and check the socket contacts
Then check the plug under the seat
Then follow the wire