Hi Snizri ,
So the initial Sync hasn't done with the standby unit.
Please Run this command in the Active unit to force initial configsync again :
open >>> tmsh
then write this command :
run cm config-sync force-full-load-push to-group <group name>
Note this is must be run in tmsh.
For details about this , have a look here : https://my.f5.com/manage/s/article/K63470472
( Or )
Try to do any change in the Active unit , something like creating a test VS or pool or node.
in sync status in Active node should be changed and you will be able to do sync from Active unit to standby.
but I recommend the first method , to force inital sync again to the device group ( the standby device ) i mean.