Ineffective Matching Rules Can’t Be Activated

Sometimes a matching rule defines matches so broadly that too many records are defined as possible duplicates. Salesforce blocks activation of such a rule and sends an email with more details to the admin who tried to activate it. This change applies to Lightning Experience, Salesforce Classic, and all versions of the Salesforce1 mobile app.
Available in: Professional, Enterprise, Performance, Unlimited, and Developer Editions

Example

Suppose that you have 1,000 accounts, all in the United States, and your rule determines whether accounts match only by looking for the same billing country. All your accounts are identified as duplicates of one another.