Getting ''chassis fpc must specify member in virtual-chassis mode'  while configuring 40g on fpc port
master:member0-re0}[edit chassis fpc 5]
lab@mx# show 
pic 0 {
   port 0 {
       speed 40g;
   }
}
lab@mx# commit check 
member0-re0: 
[edit routing-options static]
 'route 0.0.0.0/0'
   warning: requires 'l3static' license
[edit chassis fpc 5 pic 0]
 'port 0'
   chassis fpc must specify member in virtual-chassis mode
error: configuration check-out failed

Error while changing speed to 40g as 'chassis fpc must specify member in virtual-chassis mode''

In virtual chassis need to mention the member of the for which the fpc is a part.
In this example FPC 5 is part of member 0, hence correct configuration will be as below which includes member0 information
{master:member0-re0}[edit chassis]
lab@mx# show 
redundancy {
  graceful-switchover;
}
aggregated-devices {
  ethernet {
    device-count 10;
  }
}
network-services enhanced-ip;
member 0 {
  fpc 5 {
    pic 0 {
      port 0 {
        speed 40g;
      }
    }
  }
}
lab@mx# commit check 
member0-re0: 
configuration check succeeds
member1-re1: 
configuration check succeeds
member1-re0: 
configuration check succeeds
member0-re1: 
configuration check succeeds
{master:member0-re0}[edit chassis]
lab@mx# commit