In this blog you will see the solution when you are configuring the VPC , the Nexus not allow to configure the vPC.
Some of the possible cause has been discussed here :
vPC is not enabled or is not supported in the NX-OS release of software that you are running.
Ensure that the NX-OS release supports vPC.
vPC is supported in NX-OS Release 4.1 and later releases.
If the NX-OS release supports vPC, then use the command feature of vPC to enable it.
Once you will enable these feature , Nexus will allow you to configure the vPC.