That is simply because you only have one expression. There is no need to show the expression legend.
Legends will and will only show if you have more than one expressions.
In your case, if you remove the second dimension, leave only one dimension and then add the second expression (alternative expressions don't count as you can only show one expression at a time), you will see the expression legends.
Hi Fei Xu
I am not refer to display legend of my expression.
I need to display dimension competitor legend.
As there total 4 competitors at competitor field.
TDS Technology (S) P/L
Whatsapp +65 9326 1804
If I switch the two dimensions so the first dimension is competitor and the second dimension is year, it is working.
It looks that the dimension legend only shows for periods / numbers (if you change the second dimension to DATE, it will work too). Maybe you should report this to Qlik.
Attached is a work around, hope it can help?