Why SQL server maintenance plan cannot rebuild clustered index offline?

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SQL

database

This is my table structure and I select "for index types that do not support online index rebuild rebuild indexes offline" option in rebuild maintenance plan but still getting error
failed with the following error: "An online operation cannot be performed for index 'PK_Table1_1' because the index contains column 'FileContent'"; of data type text, ntext

Table structure :

CREATE TABLE [dbo].[Table1]
( [ColumnID] [INT] NOT NULL,
 [ColumnName] [NVARCHAR](250) NOT NULL,
 [FileContent] [VARBINARY](MAX) FILESTREAM NOT NULL,
 [ColumnDate] [VARCHAR](50) NOT NULL,
 [UserID] [VARCHAR](50) NOT NULL, 
[DefaultColumn] [BIT] NOT NULL, 
[ID] [UNIQUEIDENTIFIER] ROWGUIDCOL NOT NULL 
CONSTRAINT [DF__TblCheque__ID__398D8EEE] DEFAULT (NEWID()), [ReadOnly] [BIT] NOT NULL,
CONSTRAINT [PK_Table1_1] PRIMARY KEY CLUSTERED ( [ColumnID] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF,
IGNORE_DUP_KEY = OFF, 
ALLOW_ROW_LOCKS = ON, 
ALLOW_PAGE_LOCKS = ON) ON [PRIMARY] FILESTREAM_ON [FileStreamG], 
CONSTRAINT [UQ__Table1__3214EC26C89CBB09] UNIQUE NONCLUSTERED ( [ID] ASC )WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, 
IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, 
ALLOW_PAGE_LOCKS = ON, 
FILLFACTOR = 70) ON [PRIMARY] ) ON [PRIMARY] FILESTREAM_ON [FileStreamG]

What I have tried:

select au.*
 from sys.system_internals_allocation_units au
 join sys.system_internals_partitions p on au.container_id = p.partition_id
 where p.object_id = object_id('dbo.Table1');

allocation_unit_id	type	 type_desc	           container_id	
----------------------------------------------------------------------------------
72057594055688192	1	     IN_ROW_DATA	       72057594048348160
72057594055753728	3	     ROW_OVERFLOW_DATA     72057594048348160 
72057594055819264	1	     IN_ROW_DATA 	       72057594048413696