So you would have:

type ErrPermissionNotAllowed []Permission
func (perms ErrPermissionNotAllowed) Is(permission Permission) bool {
    for _, p := range perms {
        if p == permission { return true }
    }
    return false
}
var err error = errPermissionNotAllowed{"is-noob"}

if errors.Is(err, ErrPermissionNotAllowed{}) { ... } // user is not allowed

var e ErrPermissionNotAllowed
if errors.As(err, e) && e.Is("a-noob") { ... } // user is not allowed because they are a noob.