When I attempt to add a polygon with map notes (Insert Tab-Layer Templates) I get the message C:\Program Files\ArcGIS\Pro\Resources\LayerTemplates\Gallery\en-US\Simple_3_Polygon_MapNotes.lpkx. The operateion was attmpted on empty geometry.
I can navigate to that layer package. It opens and I can use Map Notes in that project, but not in my current one.
You can add the layer package through 'Add Data'. Another option is to create a folder connection to that folder, then you can add them as you see fit. Once you make that folder connection to the project, the gallery should register the tools. That is what worked for us. Hope that helps.