Frosting,
I tried the following in SSMS and got:
declare @PATIENT_NAME varchar(30) = 'abcd'
--
-- Try whole expression
--
select LEFT( LEFT(@PATIENT_NAME, (LEN(@PATIENT_NAME) - (LEN(@PATIENT_NAME)- CHARINDEX(' ', @PATIENT_NAME) + 1))) , 3)
Msg 537, Level 16, State 2, Line 6
Invalid...