When working with entity selections, recall that the expression entitySelection[index] cannot be assigned to any value.

C_OBJECT($selection_e;$ent_e) $selection_e:=ds.Table_1.all() $ent_e:=ds.Table_1.new() $selection_e[3]:=$ent // invalid assignment

Instead, assign a variable to the entity at that particular index and modify the attributes from that variable. Also make sure that the index is within range of the entity selection.

C_OBJECT($selection_es;$ent_e;$status_o) C_LONGINT($selectionLength_l;$pos_l) $selection_es:=ds.Table_1.all() $pos_l:=3 // index of entity that will be changed If ($pos_l<$selection_es.length)    $ent_e:=$selection_es[$pos_l]    // make changes to the entity here End if