VNAV and LNAV not selectable

[Mal1 0]

Hello guys,

just an short question:

 

I fly my 737-800ZIBO from "cold and dark". During programming the FMC I type in the IAS/MACH in the MCP and turn ON F/D and A/T. Then I want to preselevt the LNAV and VNAV.

 

Abd there's my problem: Why are these buttons without function? After Takeoff they work.

 

Thanks for your answers.

Mal1