I am getting SQL error msg 116, level 16, state 1, line 7 with this query I am writing

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I am getting the Error "Only one expression can be specified in the select list when the subquery is not introduced with EXISTS." with this query. I understand it has something to do with the INNER JOIN, but I do not know how to fix it.

SELECT (Grp.name) AS Wayside, (Grp.state) AS ST,
( select 'G' 
			+ SUBSTRING(CONVERT(VARCHAR(100), CAST(wea AS DECIMAL (38, 2))), 3, 3) + '/' 
			+ SUBSTRING(CONVERT(VARCHAR(100), CAST(WEA AS DECIMAL (38,2))),6,3)
			, Base1 AS 'Primary', AVG(ssi1) OVER (PARTITION BY wea,Base1, CONVERT(DATE,Date_time)) as Primary_SSI
			FROM RT_Group_Status
) AS 'Group_ID', 
(SELECT [Name] + ',' + [State]
 FROM [nms_cfg4].[dbo].[Base_Equipment]
 WHERE Base_Equip_Address = (SELECT TOP 1
                             Base_Equip_Address
                             FROM [nms_cfg4].[dbo].[be_xref_oa]
                             WHERE x_pbase = master.dbo.ufnStringToPbase([base1])
                             )
 ) + '(' + [base1] + ')' AS 'Primary',
( SELECT    [Name] + ',' + [State]
FROM [nms_cfg4].[dbo].[Base_Equipment]
WHERE  Base_Equip_Address = (SELECT TOP 1
                             Base_Equip_Address
                             FROM     [nms_cfg4].[dbo].[be_xref_oa]
                             WHERE    x_pbase = master.dbo.ufnStringToPbase([base2])
                             )
) + '(' + [base2] + ')' AS 'Secondary'
FROM [nms_rt].[dbo].[RT_Group_Status] AS Cov
INNER JOIN [nms_cfg4].[dbo].[ATCS_Group] As Grp ON Grp.Group_Address = Cov.[WEA]
ORDER BY Wayside

What I have tried:

This is my original query which works; I am trying to add an averaging of one column to this query.

SELECT  ( Grp.name ) AS Wayside, ( Grp.state ) AS ST,
        ( SELECT    'G'
                    + SUBSTRING(CONVERT(VARCHAR(100), CAST(WEA AS DECIMAL(38, 2))), 3, 3) + '/'
                    + SUBSTRING(CONVERT(VARCHAR(100), CAST(WEA AS DECIMAL(38, 2))), 6, 3)
        ) AS 'Group_ID',
        ( SELECT    [Name] + ',' + [State]
          FROM      [nms_cfg4].[dbo].[Base_Equipment]
          WHERE     Base_Equip_Address = ( SELECT TOP 1
                                                    Base_Equip_Address
                                           FROM     [nms_cfg4].[dbo].[be_xref_oa]
                                           WHERE    x_pbase = master.dbo.ufnStringToPbase([base1])
                                         )
        ) + '  ( ' + [base1] + ')' AS 'Primary',
        CAST(Cov.Average_SSI1 as VARCHAR(3)) as 'Primary_SSI',
        ( SELECT    [Name] + ',' + [State]
          FROM      [nms_cfg4].[dbo].[Base_Equipment]
          WHERE     Base_Equip_Address = ( SELECT TOP 1
                                                    Base_Equip_Address
                                           FROM     [nms_cfg4].[dbo].[be_xref_oa]
                                           WHERE    x_pbase = master.dbo.ufnStringToPbase([base2])
                                         )
        ) + '  ( ' + [base2] + ')' AS 'Secondary',
        CAST(Cov.Average_SSI2 as VARCHAR(3)) as 'Secondary_SSI',
        CAST([Date_Time] AS DATE) AS Date
FROM [NMS_RT].[dbo].[RT_Group_Average] AS Cov
INNER JOIN [nms_cfg4].[dbo].[ATCS_Group] AS Grp
          ON Grp.Group_Address = Cov.[WEA]
WHERE Date_Time >= DATEADD(DAY, -1, GETDATE())
ORDER BY Date, Wayside

Thanks,