DbConcurrencyException, what am I missing?

 public async Task SetVariable(Guid productId, Guid userId, string requestKey, string requestValue)
    {
        var product = await productRepository.GetByIdAsync(productId);
        if (product is null)
        {
            throw new ProductNotFoundException(productId);
        }

        var user = await userRepository.GetByIdAsync(productId, userId);
        if (user is null)
        {
            throw new UserNotFoundException(userId);
        }

        user.SetVariable(requestKey, requestValue);
        userRepository.Update(user);
        await unitOfWork.SaveChangesAsync();
    }

    public ICollection<UserVariable> Variables { get; set; }

    public void SetVariable(string key, string value)
    {
        ArgumentException.ThrowIfNullOrWhiteSpace(key);

        var existingVariable = Variables.FirstOrDefault(v => v.Key.Equals(key, StringComparison.OrdinalIgnoreCase));

        if (existingVariable != null)
        {
            existingVariable.UpdateValue(value);
        }
        else
        {
            var newVariable = UserVariable.Create(Guid.NewGuid(), Id, key, value);
            Variables.Add(newVariable);
        }
    }

    public void Update(User user)
    {
        dbContext.Set<User>().Update(user);
    }