The error is a bit of a red herring.
The error comes from the fact that Kentico was looking for the Default Path for the content type, instead of the Page Template's custom path.
The reason it was looking for that instead of the Page Template is because in XbyK you MUST add the content type to the AddKentico(features => features.UsePageBuilder(new PageBuilderOptions )) array.
builder.Services.AddKentico(features => {
features.UsePageBuilder(new PageBuilderOptions {
ContentTypeNames =
[
// Enables Page Builder for content types using their generated classes
Home.CONTENT_TYPE_NAME,
BasicPage.CONTENT_TYPE_NAME,
Generic.Account.CONTENT_TYPE_NAME, // THIS WAS MISSING and causing the error
],
...
});
...
});