Configure HA on trial environment

Snizri It seems like you just have to force the initial config sync since the devices are in sync and the correct active/standby status with no communication errors between them.

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.