While we have a lot of fun slang terms here, a slightly more formal term (that could still be used casually!) would be Incompetent:

a. Of inadequate ability or fitness; not having the requisite capacity or qualification; incapable.

B. An incompetent person.

Incompetent springs to mind:

a. Of inadequate ability or fitness; not having the requisite capacity or qualification; incapable.

B. An incompetent person.

While we have a lot of fun slang terms here, this would be a term you could use either casually or in a slightly more formal setting.

I've also seen "Incompetent" used as an insult ("Are you incompetent, boy?"), so it would work to be more self-deprecating.