Sql Group by on subquery rows
Rather than subqueries, use conditional aggregation:
SELECT
      DateArret
    , SUM(CASE EnumArret WHEN 'HI' THEN DATEDIFF(minute,HeureDebut, HeureFin) ELSE 0 END) AS HI
    , SUM(CASE EnumArret WHEN 'PS' THEN DATEDIFF(minute,HeureDebut, HeureFin) ELSE 0 END) AS PS
FROM dbo.ArretProductionJournee
WHERE EnumArret IN('HI', 'PS')
GROUP BY
    DateArret
ORDER BY
    DateArret;

Below is more complete sample data and schema to match your desired result.
CREATE TABLE dbo.ArretProductionJournee(
     ID int NOT NULL IDENTITY CONSTRAINT PK_ArretProductionJournee PRIMARY KEY
    ,DateArret date NOT NULL
    ,HeureDebut datetime2(3) NOT NULL
    ,HeureFin datetime2(3) NOT NULL
    ,EnumArret char(2) NOT NULL
);
INSERT INTO dbo.ArretProductionJournee(DateArret, HeureDebut, HeureFin, EnumArret)
VALUES
     ('2020-10-30', '2020-10-30 15:00:00.000', '2020-10-30 15:05:00.000', 'HI')
    ,('2020-10-30', '2020-10-30 15:00:00.000', '2020-10-30 15:07:00.000', 'HI')
    ,('2020-11-30', '2020-11-30 14:00:00.000', '2020-11-30 15:00:00.000', 'PS')
    ,('2020-11-30', '2020-11-30 16:00:00.000', '2020-11-30 17:00:00.000', 'HI')
    ,('2020-11-30', '2020-11-30 16:00:00.000', '2020-11-30 17:00:00.000', 'XX');