resource string/app_name (aka com.example.calculator:string/app_name) not found.

This error message indicates that the resource string with the identifier "app_name" could not be found in the specified package (com.example.calculator). It is possible that the string was not defined in the strings.xml file or that it was misspelled in the code. To fix this error, ensure that the string is defined correctly in the strings.xml file and that the package name is correct in the code.