You start the IF THen Else with a number and finish on a While, I have seen this before, expressions both sides of the IF Then Else must be the same.

Try splitting into two separate statements eg

EvaluateAfter ({@issuer_test});
numberVar LengthIssuer1 ;
global stringvar issuer1_for_test;
global stringVar array  issuer1_part:=['a','b','c','d','e'];
numbervar i:=1;
numbervar j:=1;
numbervar l:=0;

LengthIssuer1:=Length (issuer1_for_test);

If LengthIssuer1 <=26 then
(
    issuer1_part[1]:=issuer1_for_test;
    j:=j;
    
);


(

      while j<= LengthIssuer1 do
      (
        if  mid({@issuer_test},j+26,1)=' ' then
        (     
            issuer1_part:=mid({@issuer_test},j,26);
            j:= j+ 27;
            i:=i+1
        )
     )
);

ie replace else with a ; and add a ; at the end of the second part.

Not sure why this happens but splitting like this has worked on simpler formulas for me.

Ian