One or more devices are unreachable. Resolve any communication problems before attempting to sync.

Hi,

Can you please run some TMSH commands and paste the output?

tmsh list net vlan 
tmsh list net self 
tmsh list cm device  | grep -v modules

Lastly, initiate a config sync, despite the warning, and paste the relevant lines from the /var/log/ltm file.

Have you tried following existing SOLs? One relevant article is here: https://support.f5.com/kb/en-us/solutions/public/13000/900/sol13946.html