Error when try a "cast" in a char(13) to get a varchar....

Question

AVAB_Codigo it's a "char(13)", get me a error :
"Invalid operator for data type. Operator equals boolean AND, type equals varchar."
when i use a cast...  the error still appear,.....  what should i do ?
-- 1st attemp --------------------------------------------------------------------
  case when (SUBSTRING(AVAB_Codigo,5,1)='0') 
   then Right(AVAB_Codigo,6) 
   else (SUBSTRING(AVAB_Codigo,5,1) & Right(AVAB_Codigo,6)) 
  end as [UL/UE]
-- 2nd attempt ----------------------------------------------------------------------
  case when (SUBSTRING(cast(AVAB_Codigo as varchar(10)),5,1)='0') 
   then Right(cast(AVAB_Codigo as varchar(10)),6) 
   else (SUBSTRING(cast(AVAB_Codigo as varchar(10)),5,1) & Right(cast(AVAB_Codigo as varchar(10)),6)) 
  end as [UL/UE],

Answer 1

The concatenation operation is the plus sign, not the ampersand.