>Now I am using like this
>
>

>Index On acc_code+dtoc(Date,1) TO SYS(2015)
>

>
>
>But same error.

If acc_code is not a string, try

Index On padr(acc_code,20)+dtoc(Date,1) TO SYS(2015)

The length of 20 is there just in case - depending on the size of your numbers in that column, perhaps only 8 or 6 would do.