'Settings' is inaccessible due to its protection level

This error is because of Settings class is internal.

I assumed that you have created and modified settings from visual studio project properties section settings. Right click on project > Properties > Settings. There is a drop down menu called Access Modifier that you need to change from internal to public.

For more information on internal keyword: The internal keyword is an access modifier for types and type members. Internal types or members are accessible only within files in the same assembly