将主键拖放到单独的迁移中，然后删除 $ table-> primary('id').添加 AUTO_INCREMENT 列会自动创建一个主键:

Drop the primary key in a separate migration and remove $table->primary('id'). Adding an AUTO_INCREMENT column automatically creates a primary key:

Schema::table('item_tag', function (Blueprint $table) {   
    $table->dropPrimary();
});

Schema::table('item_tag', function (Blueprint $table) {
    $table->unsignedInteger('id', true)->first();
});

您还可以简化第二次迁移:

You can also simplify the second migration:

Schema::table('item_tag', function (Blueprint $table) {
    $table->increments('id')->first();
});