xUnit test EF throws an exception c#

During the test, two EF methods throw an exception.I kind of made a mock object. What is the problem?

Command Test

    public class CommandUserTests
    {
        private readonly Mock<IStoreDbContext> _context;
        private readonly Mock<IUserStore<User>> _userManager;
        public CommandUserTests()
        {
            _context = new Mock<IStoreDbContext>();
            _userManager = new Mock<IUserStore<User>>();
        }

        [Fact]
        public async Task CreateUserHandler_Success()
        {
            //arrange
            var fixture = new CreateUser.Command("pavel", "[email protected]", "papsdadk21xi", "Poland", "+3753391822183");
            var handler = new CreateUser.Handler(_context.Object,_userManager.Object);

            //act
            var result = await handler.Handle(fixture, new CancellationToken());

        }

Command

    public static class CreateUser
    {
        public record Command(string UserName, string Email, string Password, string Country,
            string PhoneNumber) : IRequest<Guid>;

        public class Handler : IRequestHandler<Command, Guid>
        {
            private readonly IStoreDbContext _context;
            private readonly IUserStore<User> _userManager;

            public Handler(IStoreDbContext context, IUserStore<User> userManager)
            {
                _context = context;
                _userManager = userManager;
            }

            public async Task<Guid> Handle(Command request, CancellationToken cancellationToken)
            {
                var Country =
                    await _context.Countries.FirstOrDefaultAsync(c => c.Name == request.Country, cancellationToken);//throw exception 
                //
                var user = new User
                {
                    Id = Guid.NewGuid(),
                    UserName = request.UserName,
                    Email = request.Email,
                    PasswordHash = request.Password,
                    Country = Country,
                    CountryId = Country.Id,
                    PhoneNumber = request.PhoneNumber,
                    CreateDate = DateTime.Now
                };

                var users = new List<User>();
                users.Add(user);
                Country.Users = users;
                _context.Countries.Update(Country);//throw exception 
                //
                await _userManager.CreateAsync(user,cancellationToken);
                await _context.SaveChangesAsync(cancellationToken);
                return user.Id;
            }
        }
    }