Добавьте файлы проекта.

Yuna.Website/Yuna.Website.Server/Services/UserService/IUserService.cs

@@ -0,0 +1,16 @@
+using System.Collections.Generic;
+using Yuna.Website.Server.Model;
+
+namespace Yuna.Website.Server.Services.UserService
+{
+    public interface IUserService
+    {
+        public Task<User?> GetById(long id);
+        public Task<User?> GetByUsername(string username);
+        public Task<IReadOnlyList<User>> GetList();
+        public Task<User?> Create(User user);
+        public Task<User?> Update(User user);
+        public Task<User?> Delete(long id);
+
+    }
+}