On EX4400-24X switch when creating a virtual-chassis ports on 2x100G front panel ports, 4x25G uplink module, and the 1x100G uplink module, the switch does not allow to manually specify the VC-Port

If we manually specify VC-port

user@switch@. request virtual-chassis vc-port set pic-slot <slot number> port <port number>
"error: Invalid PIC slot / PIC Port" 

Check VC-port status "show virtual-chassis vc-ports"

root> show virtual-chassis vc-port
fpc0:
--------------------------------------------------------------------------
<no-output>

By default, none of the ports operate in VC mode when the system comes up initially, and the 2x100G front panel ports operate as network ports by default. To confirm, execute 

user@switch> show virtual-chassis mode
fpc0:
--------------------------------------------------------------------------
Current mode :  network-port mode

Convert Network-port to HGoE port

user@switch. request virtual-chassis mode hgoe reboot

Verify port mode  

user@switch> show virtual-chassis mode
fpc0:
--------------------------------------------------------------------------
Current mode : Virtual Chassis with hgoe mode enabled

Configure the Front panel port to be used as VC-Port

user@switch> request virtual-chassis vc-port set pic-slot <slot number> port <port>
 

NOTE: 

• Starting with Junos version 23.1R1.8
• Front Panel Port (Uplink / Stacking Port)
• EX4400-24X switches support virtual-chassis only on the HGoE mode and these switches do not support HiGiG mode.

Referral Document: https://www.juniper.net/documentation/us/en/software/junos/virtual-chassis-qfx/topics/concept/virtual-chassis-ex4200-overview.html