SQL Server Data type error

Hi Experts,

I am facing an data type error while running below query. Please advise how to fix below error.

SELECT  Inquiries,[PO Number], max(Revision) AS MaxRevision
FROM [LinkedServer_1].[Database_1].dbo.CPOA  WITH(NOLOCK)
GROUP BY Inquiries ,[PO Number]

Error:
Msg 306, Level 16, State 2, Line 3
The text, ntext, and image data types cannot be compared or sorted, except when using IS NULL or LIKE operator.

Data type:
---------
Inquiries varchar(max)
[PO Number] varchar(50)
Revision varchar(50)

Source server version:
Microsoft SQL Server 2008 R2 (SP2) - 10.50.4000.0 (X64)   Jun 28 2012 08:36:30   Copyright (c) Microsoft Corporation  Enterprise Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1) (Hypervisor)

Destination version (lnked server version):
Microsoft SQL Server 2008 (SP3) - 10.0.5500.0 (X64)   Sep 21 2011 22:45:45   Copyright (c) 1988-2008 Microsoft Corporation  Enterprise Edition (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1)