The message is telling you that the doc_revision property on the target item revision, "ALNX/00016/03", does not match the handler's -value argument of "00".
What I do is put that handler on the start step at the start task. It will do a check first, if the check failed, you can't even start the workflow.
Any experience person can tell me if it is the right? Or I should have validation task for it specifically?
If you put it in the start task, this will happen. It's a good way to restrict people from starting a workflow with wrong kind of targets or or incomplete targets. Teamcenter will handle the "error" by not executing the workflow.
If you put the handler in a subsequent task, the workflow will start and the "error" will happen later on. If you do not handle the error properly by creating an error path the users will get confused because the outcome is a failed workflow process. Add an error path, and you have a way to branch your workflow.