I just had this issue when using TOPCAT 2.74Beta to calculate my derate.  On the Flaps -> Optimum setting, it recommended Flaps 20 for me, which the 777 doesnt like apparently.  I had TOPCAT recalculate using Flaps 15 specifically and the FMC gave me v-speeds.  

Which B777? TOPCAT does not have 300ER performace tables, which is the reason I ask.

 

I've used F-20 TOPCAT and 200LR on several occasions with no problem.

 

By the way, please use your full name in posts to the PMDG forum, thanks.