I've downloaded the latest version of the attribute assistant from here:
Requirements says: ArcMap (Standard or Advanced) 10.2.1, 10.5 - 10.8
I get the following error:
I've tried running ArcMap 'as administrator'... same result.
Just in case anyone else runs across this ... apparently you can't add the extension via the 'Addin Manager/Customize' dialog.
Just double-click/execute the add-in with all the arcgis apps closed. As per the doc: