Do you have multiple umbraco nuget packages installed?

Umbraco 14 does not use startup.cs just program.cs which OOB looks like

WebApplicationBuilder builder = WebApplication.CreateBuilder(args);

builder.CreateUmbracoBuilder()
    .AddBackOffice()
    .AddWebsite()
    .AddDeliveryApi()
    .AddComposers()
    .Build();

WebApplication app = builder.Build();

await app.BootUmbracoAsync();


app.UseUmbraco()
    .WithMiddleware(u =>
    {
        u.UseBackOffice();
        u.UseWebsite();
    })
    .WithEndpoints(u =>
    {
        u.UseBackOfficeEndpoints();
        u.UseWebsiteEndpoints();
    });

await app.RunAsync();

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