Is it possible to load the effective address in SI/DI with the string and a variable?

Question

DATA SEGMENT
    STR DB "HELLO!!$"
    LEN DB ?
DATA ENDS

CODE SEGMENT
ASSUME CS:CODE,DS:DATA
START: MOV AX,DATA
       MOV DX,AX
       MOV BL,STR+1
       MOV LEN,BL
       LEA SI,STR+LEN
       CODE ENDS
       END START

Here, I want to just to show an example which I wrote the above code, actually, I am trying this technique to implement in code palindrome of a string. But, however, it shows the following error: error output

Hence, I want to know how can I implement it in some other way to achieve it??