Validate item numbers during item creation

We have a requirement where we are trying to validate Item Numbers. We have a combination of Items - a base item and its corresponding service item. The requirement is that creation of service item must be allowed only if base item exists.

For example, lets consider base item to be 12345 and its corresponding service item would be 12345-SVC. The validation condition on the item rule will check if item 12345 exists in the system and only then would it allow creation of item 12345-SVC

In order to achieve this, we have created item rule, however it is throwing a syntax error as shown. Could you please educate on this? Would validation rules help at all in this scenario? Or what are the options do we have?