How to byte characters on mobile keyboard?

I have been trying to enter something in a Gunpla Builders World Cup regulation entry, but the system said that two-byte characters were used and that I have to use one-byte characters. Is there a way to change my keyboard on my mobile phone so that I can write in one-byte characters?

Have you tried looking for an online converter to change it to two-byte? There are usually lots of ways to change it, even without an app.