Once an attribute rule is saved in an ArcGIS Pro project, it seems that what you have is what you are stuck with. A common issue I have encountered is not checking the Exclude from application evaluation under the Execution options. I don't quite understand it , but often times this needs to be checked. If you create a rule and save it, then test it, you may get this error:
However, if you go over to the attribute rules pane, you encounter:
Where the option Exclude from application evaluation is grayed out. So, you need to delete the rule add it back in and check that box before you save.
Can we have it so the Execution options are available with out deleting the rule and starting over? Maybe even make them available if you disable the rule?