Error AssemblyInfo.cs can't find type or namespace 'Core'

I'm not sure of what's going on here, even if creating an indicator from start it throws this error, however I replace the code somewhere else (an old indicator) and it works even if the code is the same, I'm out of clues, where's this namespace 'Core' located?.

This is a simple indicator, it plots two ATR's.

using System;
using cAlgo.API;
using cAlgo.API.Internals;
using cAlgo.API.Indicators;
using cAlgo.Indicators;

namespace cAlgo
{
    [Indicator(IsOverlay = false, TimeZone = TimeZones.UTC, AutoRescale = false, AccessRights = AccessRights.None)]
    public class DoubleATR : Indicator
    {

        [Output("Main", Color = Colors.Pink, IsHistogram = false, LineStyle = LineStyle.Solid, PlotType = PlotType.Line, Thickness = 1)]
        public IndicatorDataSeries _Wide { get; set; }
        [Output("Short", Color = Colors.CornflowerBlue, IsHistogram = false, LineStyle = LineStyle.Solid, PlotType = PlotType.Line, Thickness = 1)]
        public IndicatorDataSeries _Short { get; set; }

        private AverageTrueRange _WideATR, _ShortATR;

        protected override void Initialize()
        {
            _WideATR = Indicators.AverageTrueRange(100, MovingAverageType.Simple);
            _ShortATR = Indicators.AverageTrueRange(14, MovingAverageType.Simple);
        }

        public override void Calculate(int index)
        {
            _Wide[index] = _WideATR.Result.LastValue;
            _Short[index] = _ShortATR.Result.LastValue;
        }
    }
}