EF Core giving me error Cannot insert explicit value for identity column in table 'Tags' when IDENTITY_INSERT is set to OFF

My project using DDD pattern,is built by .NET-core and EF core. Today, I meet a question is that when I add a tag entity to my database, then do saveChanges. But the program throw a Exception: "Cannot insert explicit value for identity column in table 'Tags' when IDENTITY_INSERT is set to OFF" And I am puzzled, also, I search the question on the stackoverfolw and google. I found some solution is that use the attribute [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)], but is doesn't work for me. Here is my code below:

C#//Entity
public class Tags
{
    [DatabaseGeneratedAttribute(DatabaseGeneratedOption.Identity)]
    public int TagId { get; set; }
    public string TagName { get; set; }
    public TagUseCount TagUseCount { get; set; }
    public enum Operation
    {
        None = 0,
        Add = 2, 
        Update = 4
    }
}

The Function use in my Service Lawyer:

var tagEntity = new Tags { TagName = tag };
var tagId = await _tagRepository.Add(tagEntity);

The Add Function is Repository Lawyer:

public async Task<int> Add(Tags tags)
    {
        try
        {
            _unitOfWork.Add(tags);
            await _unitOfWork.CommitAsync();
            return tags.TagId;
        }
        catch (Exception ex)
        {
            _logger.LogError(new EventId(), ex, "Error when add Tag!");
        }
        return 0;
    }

UOW Add Method:

TEntity IUnitOfWork.Add<TEntity>(TEntity entity)
    {
        return base.Set<TEntity>().Add(entity).Entity;     
    }

Version of EF core :"Microsoft.EntityFrameworkCore": "1.1.0"

ModelBuilder below:

modelBuilder.Entity<Tags>().ToTable("Tags")
            .HasKey(x => x.TagId);


modelBuilder.Entity<TagUseCount>().ToTable("TagUseCount")
            .HasKey(x => x.TagId);
modelBuilder.Entity<TagUseCount>().ToTable("TagUseCount")
           .HasOne(x => x.tags)
           .WithOne(q => q.tagUseCount);

I am crazy with it for a couple of days, Who can help me ?