DRBD collector is for https://en.wikipedia.org/wiki/Distributed_Replicated_Block_Device , this may not be present on a system. So this is not an error that would be preventing server connecting to agent.
check output of
netstat -lptn
on agent's system. It should show something like
tcp 0 0 0.0.0.0:4700 0.0.0.0:* LISTEN 6347/nxagentd
that would mean that NetXMS agent is listening on port 4700.
From another machine you can try
telnet agent_machine_ip 4700
to see if it's possible to establisth network connection to agent.
In netxms console try right-clicking the node and performing Configuration Poll (full).
