Error in max function

[BHUVANESH]

The inputs for function 'MAX' should be a list of numeric values

This Error Shows The Max Function  i Will Use...

[batch serial 1] & BP & MAX(
Select
(
Batch[Batch serial 2],
[_THISROW].[batch serial 1] = [Cohort]
)
)+ 1

This is the expression ..please any one clarify