The “Unresolved Externals” error appears to be caused by the use of MainCRTStartup. I’m not sure if this is a plugin or module you’re using with your project, but can you try disabling/removing it and try to upgrade the project again? If this is a module you’re using with your project, you may need to update the Build.cs file. Additionally, let me know if you’re able to upgrade the project one version at a time (4.12 to 4.13 to 4.14 to 4.15) rather than jumping from 4.12 to 4.15.
Cheers