Hi Vaibhav,

CMS 12 supports .NET 10 and the recommended minimum version is EPiServer.Cms.UI 12.34.4, which you already have.

This error looks more related to how TinyMCE is registered or used in your custom editor descriptor testXhtmlStringEditorDescriptor I would first check if that or some older TinyMCE setup is trying to inject ServiceAccessor<TinyMceConfiguration> directly, for CMS 12 TinyMCE configuration should be registered through TinyMceConfiguration for example

public static class ServiceCollectionExtensions
{
    public static IServiceCollection AddTinyMceConfiguration(this IServiceCollection services)
    {
        services.Configure<TinyMcePropertySettingsOptions>(options =>
        {
            options.Enabled = true;
            options.AllowAddingBlocks = false;
        });

        services.Configure<TinyMceConfiguration>(config =>
        {
            config.Default().AddEpiserverSupport();
        });

        return services;
    }
}

and then register it in Startup.cs

public void ConfigureServices(IServiceCollection services)
{
    services.AddTinyMceConfiguration();
}

So I would suggest checking the custom descriptor first, then making sure all CMS UI and TinyMCE packages are aligned.

Docs: https://docs.developers.optimizely.com/content-management-system/docs/default-settings

Regards,
Wojciech