ASP.Net 控制器到服务连接错误

Question

我正在创建 Web API，但出现错误。我想从控制器开始，从控制器调用服务并从服务调用存储库。但是有些东西不起作用，我无法弄清楚。有人可以帮助我吗？

控制器

namespace uhem_api.Controllers
{
    [ApiController]
    [Route("[controller]")]
    public class TravelPurposeController : ControllerBase
    {
        private readonly ITravelPurposeService _travelPurposeService;

        public TravelPurposeController(ITravelPurposeService travelPurposeService){
            
           _travelPurposeService = travelPurposeService;
 
        }

        [HttpGet]
        public async Task<List<TravelPurposeDto>> GetAll()
        {
            var res = await _travelPurposeService.GetAll();

            return res;
        }

    }
}

服务

namespace uhem_api.Services
{
    public class TravelPurposeService : ITravelPurposeService
    {
        .
        .
        .
        .