They have no idea how to arrange their songs. It doesn't mean you won't make it. It just means that you'll need someone else to arrange all of your songs for you. But if you can't find a particular arrangement for a song, it might be a good idea to try and enlist someone else's aid. As I said, some people's jobs are to arrange songs, it's what they are good at.
Some people have a knack for it.
If you know someone like that, I suggest you stick close to them. Of course, some people will ask for money. Others may ask for a co-writing credit. Normally, you shouldn't share the credit with someone unless they make a major contribution. You may want to try and only share the composing credit and not the lyrical one.