Hi,  I have tested pretty much every command in the 787.  The system is recognizing my command "Set Heading 124" but dosen't change on the MCP, i am having the same issue with selecting an altitude

Make sure:

1. The wheels are off the ground.

2. This config option is set to YES:

ALLOW MCP CMDS WITH AP ON: YES

 

It's probably #2.  Remember when the autopilot is on, as the Pilot flying, you control the autopilot panel, not the FO.

But we have that config option for those who don't want it that way.