No matter what I do, my account (ID: 1016302879) simply won’t generate the account.dat file. Could it be because the username is too long? Any help would be appreciated!
What do you mean it won’t generate it?
If you log into your PNID on a Wii U user, it should be generated.
As Joshua has said, you need to link your PNID to a profile on a Wii U for the account.dat to be generated. You require a real, physical Wii U not just for your account.dat but also for other console identifiers which we require for online play
Your PNID is unlikely to be ‘too long’, my PNID, SammySquash9608, is 15 characters long, yours is 10 characters long