A suggestion would be
SELECT BranchName, ContractNo, Email, StartDate, EndDate,
CASE
-- notice that the 101 is my favorite see BOL Convert for list of others
WHEN CONVERT(VARCHAR, DATEADD(day, ContractReminder, GETDATE()), 101) > EndDate
THEN 'Less than ' + CAST(ContractReminder as VARCHAR) + ' days left'
WHEN CONVERT(VARCHAR, DATEADD(month, remindertype, GETDATE()), 101) > EndDate
THEN 'Less than ' + remindertype + ' months left'
END AS MyFlag
INTO #RList
WHERE CONVERT(VARCHAR, DATEADD(day, ContractReminder, GETDATE()), 101) > EndDate
OR CONVERT(VARCHAR, DATEADD(month, remindertype, GETDATE()), 101) > EndDate