How can I fix this? I would appreciate any help.
Before packaging your app, you need to test your app in .Net Native compilation environment to improve your app's runtime performance.
You could right-click your project > click property
> select Build
> check Compile with .NET Native tool chain
.
By default, the Release build configuration enables the .NET native toolchain so you could test your app in release build.
After that, you could package your app following Packaging UWP apps and upload it to Dev Center.