The cause could be the same as i mentioned above, mainly the partial renaming. It's also possible that the cause is installing ArcGIS Pro multiple times under different folders.
To verify your installation simply create a new 'ArcGIS Pro Module Add-in' name it 'ButtonTest', once the project has been created right click (in solution explorer) on the project, click 'Add | New item', and choose 'ArcGIS Pro Button'. Now open the Button1 class file and set a breakpoint at the open curly for the OnClick method. Click on 'Build | Rebuild' then debug your code. Because Pro will only load your add-in when it's needed, you will see the breakpoint Warning you mentioned, however, once you go to the 'add-in' tab in Pro and click on your 'Button1' icon on the tab, your add-in DLL will be loaded and your breakpoint will be triggered.
Let me know if this works for you.