When I execute the query shows error invalid rownum

You might need to use subquery or CTE because the ROW_NUMBER() is being calculated/applied to each row during execution. Example CTE (Common Table Expression)

WITH temp as (
	SELECT dbo.fn_parsehtml(MCS_CASE_SUMMARY)
		,ROW_NUMBER() OVER (
			ORDER BY RRH_MR_NUM
			) AS RowNum
		,RRH_LOCATION_CD
		,RRH_MR_NUM
		,RRH_FIRST_NAME
		,RRH_PAT_SEX
		,RRH_REGN_DT
		,RRH_PAT_DOB
		,MCS_CRT_DTE
	FROM MR_CASE_SUMMARY_HISTORY_2015
	LEFT OUTER JOIN RE_REGISTRATION_HEADER ON MCS_MRD_NUMBER = rrh_mr_num
	LEFT OUTER JOIN CO_PATIENT_ADDRESS_DETAIL ON RRH_COMM_ID = CPA_COMM_ID
		AND CPA_MAILING_ADDR = '1'
) SELECT * FROM temp
	WHERE RowNum BETWEEN 1 AND 10

or subquery

SELECT * FROM
(
	SELECT dbo.fn_parsehtml(MCS_CASE_SUMMARY)
		,ROW_NUMBER() OVER (
			ORDER BY RRH_MR_NUM
			) AS RowNum
		,RRH_LOCATION_CD
		,RRH_MR_NUM
		,RRH_FIRST_NAME
		,RRH_PAT_SEX
		,RRH_REGN_DT
		,RRH_PAT_DOB
		,MCS_CRT_DTE
	FROM MR_CASE_SUMMARY_HISTORY_2015
	LEFT OUTER JOIN RE_REGISTRATION_HEADER ON MCS_MRD_NUMBER = rrh_mr_num
	LEFT OUTER JOIN CO_PATIENT_ADDRESS_DETAIL ON RRH_COMM_ID = CPA_COMM_ID
		AND CPA_MAILING_ADDR = '1'
) r
	WHERE r.RowNum BETWEEN 1 AND 10

reference: Why You Can't Use ROW_NUMBER() In Your WHERE Clause - SQL Theater[^]