Does the feature class you're making an attribute rule for have any unsaved edits? Maybe someone else is editing the feature class, or you have it open with unsaved edits in another project file.
Thank you for the image. Another note here is that at ArcGIS Pro 2.5 the software will present a warning message here. but In 2.4 this message was absent.
Currently the Documentation lacks the licencing notes available in the software at 2.5, - Introduction to attribute rules—Geodatabases | Documentation
Please help me submit feedback on the documentation page to ask for the licencing notes to be added. Thank you.