Roll the features back (use "set current feature") before you attempt the boolean operation. You are picking a feature that was created after the body you are trying to edit... In some cases if you created some subtract or unite features later, even if you use set current feature, it will give that error. Try deleting the boolean features (Unite, Subtract) themselves. This is another reason why it is more desirable to have separate protrusion and boolean features, then to roll the "unite" into an "extrude for example.
-Derek
DL Engineering Services
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