When right clicking a project in Visual Studio 2015 and selecting Add > New item > ArcGIS Pro Add-ins, an error message stating "Value does not fall within expected range." is displayed. Visual Studio then presents the
Add New Item dialog box again. After clicking "OK" on the error message, the newly added control appear to be functioning normally. I googled this error quickly and saw some suggestions to delete the .user and .suo files and the bing and obj folders, however the error message still persists. Has anyone else experienced this?
I was not able to duplicate your scenario. Just to recap, you have an existing add-in project (c#) and you try to add a new add-in template item to your existing project. At the point right after you "drill down" to the "ArcGIS Pro Add-ins" item templates selection list you get an error pop-up? I tried this exact scenario and didn't get the error message. Just to make sure that the setup worked properly can you please try the following and then retest:
I am having a similar experience in Visual Studio 2017. I have uninstalled and reinstalled the SDK extensions which had no effect, and then did a visual studio repair which also had no effect. The one thing I noticed (which may not be related) is that when adding an SDK item such as a button, the icons that are added to the project have their "Build Action" property set to "Resource" instead of "AddInContent".
Other than that, there doesn't appear to be any problems with the created classes.