Usually the issue gets resolved after replacing the sylink.
Make sure that the system is there in the network or not. If it is there just replace the sylink
The sylink.xml file being used by one or more clients refers to a Domain ID that no longer exists. This could occur if the Endpoint Manager was reinstalled or the former Domain deleted.
Replace the sylink.xml file with one that references a valid Domain ID.
Copy of the file Sylink.xml from the server from C:\Program Files\Symantec\Symantec Endpoint Protection Manager\data\outbox\agent\
2. On the client computer , click Start > Run, type smc -stop, and click OK.
3. Copy the Sylink.xml into the C:\Program Files\Symantec\Symantec Endpoint Protection folder, and replace any existing Sylink.xml file.