This article describes if FEC108 is supported or not on EVO-QFX series.

Unable to configure FEC108 on QFX5130/QFX5220:

[edit]
root@c15-20# run show version | match :                     
Hostname: c15-20
Model: qfx5220-128c
Junos: 23.4R2-S5.4-EVO
Yocto: 3.0.2
Linux Kernel: 5.2.60-yocto-standard-gd0a736a
 
 
root@c15-20# set interfaces et-0/0/0:0 ether-options fec ? 
Possible completions:
  fec74                FEC74 enabled
  fec91                IEEE 802.3bj Clause 91, Reed-Solomon FEC (RS-FEC)
  none                 FEC disabled

[edit]
root@c15-20# set interfaces et-0/0/0:0 ether-options fec108
                                                     ^
syntax error.

[edit]
root@c15-20# 
 

For QFX5130 it only mentions that supports the following FEC settings : CL74 FEC support for 25-gigabit and 50-gigabit channel speeds

QFX5130

https://apps.juniper.net/feature-explorer/select-platform.html?typ=1&category=Switching&pid=31705133&platform=QFX5130-32CD&swName=Junos%20OS%20Evolved&rel=23.2R1&sid=1278&swtab=Junos%20OS%20Evolved#:~:text=Port%20Speed-,Auto%2Dnegotiation%20and%20port%20speed%20(configurable),43695,-Management%20Interface

The following link for QFX5220 , does not mention about FEC.

https://apps.juniper.net/feature-explorer/select-platform.html?typ=1&category=Switching&pid=31705228&platform=QFX5220-128C&swName=Junos%20OS%20Evolved&rel=23.4R2&sid=1363&swtab=Junos%20OS%20Evolved

FEC108 is not supported on EVO-QFX, we suggest reaching out your Juniper Account Team to open an Enhancement Request.