How to tell Entity Framework that my ID column is auto-incremented (AspNet Core 2.0 + PostgreSQL)?

You have to use here "ValueGenerationOnAdd()". As the issue you are getting is already reported on GitHub. Please find the below link.

https://github.com/npgsql/Npgsql.EntityFrameworkCore.PostgreSQL/issues/73

You can find more info regarding Generated Value pattern from following link.

Value generated on add

public classs SampleContext:DBContext{
public DbSet<Tag> Tag { get; set; }

protected override void OnModelCreating(ModelBuilder modelBuilder){
    modelBuilder.Entity<Tag>()
        .Property(p => p.ID)
        .ValueGeneratedOnAdd();
 }
public class Tag{
  public int Id { get; set; }
  public string Name { get; set; }
  public string Description{get;set;}
  }
}

Source:- https://www.learnentityframeworkcore.com/configuration/fluent-api/valuegeneratedonadd-method

Hope this will help

According to the Postgre SQL docs here is an example which shows how to achieve the desired result:

protected override void OnModelCreating(ModelBuilder modelBuilder)
   => modelBuilder.Entity<Blog>().Property(b => b.Id).UseIdentityAlwaysColumn();