I find this whole thing frustrating, why can I still not get my name to be "Paul (sabret00the)". I write in the first name box Paul in the nickname box sabret00

I find this whole thing frustrating, why can I still not get my name to be "Paul (sabret00the)". I write in the first name box Paul in the nickname box sabret00the and then I can't remove the holding letter I applied previously. I then try and simply remove the W and I'm told I need to have my full name up. Why can't I just have Paul and in brackets sabret00the?