Error 20598 - The row was not found at the Subscriber when applying the replicated command
What may be the possible reason of it?


There are multiple reasons that could cause this:


It could be someone or a process/apps deleted and inserted the data in subscriber db (while cmds are being applied). Review your subscriber security settings and check what process/apps are accessing the subscriber db. 
Multiple publications connected to subscriber db. pub1 (deleted the data first) then pub2 tried to UPDATE/DELETE the data which could cause an error 20598.
Triggers on subscriber tables that could delete/insert/update the data.


HTH