aspnet core unable to resolve dependency: error:'Unable to resolve service for type while attempting to activate'

I am getting a dependancy injection error when i try to access my controller. the error is 'unable to resolve service for IOrderPartRepository while attempting to activate. I followed examples from tutorials, but am still not sure why it won't resolve. I reviewed similar questions, but it looked like i had incorporated the fix from similar questions in my solution

Error: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[3] Route matched with {action = "GetOrderParts", controller = "OrderPart"}. Executing controller action with signature System.Threading.Tasks.Task`1[Microsoft.AspNetCore.Mvc.IActionResult] GetOrderParts(DatingApp.API.Helpers.UserParams) on controller DatingApp.API.Controllers.UsersController (DatingApp.API). info: Microsoft.AspNetCore.Mvc.Internal.ControllerActionInvoker[2] Executed action DatingApp.API.Controllers.UsersController.GetOrderParts (DatingApp.API) in 3.2277ms fail: Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware[1] An unhandled exception has occurred while executing the request. System.InvalidOperationException: Unable to resolve service for type 'DatingApp.API.Data.IOrderPartRepository' while attempting to activate 'DatingApp.API.Controllers.OrderPartController'.

Here is the code

public interface IOrderPartRepository
{
     void Add<T>(T entity) where T: class;
     void Delete<T>(T entity) where T: class;
     Task<bool> SaveAll();
     Task<PagedList<OrderPart>> GetOrderParts(UserParams userParams);
     Task<OrderPart> GetOrderPart(int id);
}

public class OrderPartRepository: IOrderPartRepository
{
    private readonly DataContext _context;


    public OrderPartRepository(DataContext context)
    {
        _context = context;
    }
    public void Add<T>(T entity) where T : class
    {
        _context.Add(entity);
    }

    public void Delete<T>(T entity) where T : class
    {
        _context.Remove(entity);
    }

    public async Task<OrderPart> GetOrderPart(int id)
    {
        var orderPart = await _context.OrderParts.Include(p => p.Photos).FirstOrDefaultAsync(u => u.Id == id);

        return orderPart;
    }

    public async Task<PagedList<OrderPart>> GetOrderParts(UserParams userParams)
    {
        var orderparts = _context.OrderParts.Include(p => p.Photos)
            .OrderByDescending(u => u.Added).AsQueryable();


        if (!string.IsNullOrEmpty(userParams.OrderBy))
        {
            switch (userParams.OrderBy)
            {
                case "created":
                    orderparts = orderparts.OrderByDescending(u => u.Added);
                    break;
                default:
                    orderparts = orderparts.OrderByDescending(u => u.Added);
                    break;
            }
        }

        return await PagedList<OrderPart>.CreateAsync(orderparts, userParams.PageNumber, userParams.PageSize);
    }

    public async Task<bool> SaveAll()
    {
        return await _context.SaveChangesAsync() > 0;
    }

}
}

controller

private readonly IOrderPartRepository _repo;

public OrderPartController(IOrderPartRepository repo)
{
   this._repo = repo;
}

startup

services.AddScoped<IAuthRepository, AuthRepository>();
services.AddScoped<IDatingRepository, DatingRepository>();
services.AddScoped<IOrderPartRepository, OrderPartRepository>();