I made a functionalized graphene by adding some functional groups to pristine graphene.  After I use Calculate Bond Types, I cann't run geometry optimization with force field PCFF. 

I saw some errors: 

Unable to calculate the ForcefieldType for particle 'C'
The typing engine has been unable to assign a forcefield type.
No typing rules found that match (except the wildcard '?').

...

The typing engine has failed to calculate a forcefield type for 60 particles.
Try using a different forcefield or assign types manually (and remember to turn off the automatic calculation of forcefield types).

Could  you tell me what the error's reason is? And How to fix that?

Thank you very much.