1. The rule resolution fails due to the target property being in a higher ruleset version or a class that's not accessible (If the data transform is in class Pega-Data and the property is in Pega-Data-Customer).
2. The Ruleset of the target property is higher in the hierarchy of the Application Ruleset Stack. For example, if you have the data transform in Ruleset XYZ-ABC 01.01.01 and the Target property is in MyOrg-Data 01.01.01 and the Application Ruleset Stack has the MyOrg Ruleset above the XYZ Ruleset.
Both these scenarios will generate the same error as you mentioned.
If you can mention your application ruleset stack and the class of the data transform and the property, I could probably point at your exact issue.