General

[quote=sangjae321]so i magnify my shield and it came out only bonus potential. what happen if i cube it? does bonus disappear or does it create a new line?[/quote]

Normal cubes don't affect bonus potential.
Bonus potential is completely separate from normal potential.

As the others said, since your item doesn't have any normal potential, it can't be cubed now.
After you use a potential scroll on it, it will have normal potential lines. Adding normal potential won't change the bonus potential it now has.
Then you can use normal cubes to change the normal potential lines. Again, cubing the normal potential won't change the bonus potential lines.
The only way to change the bonus potential lines is to use special Cash Shop items. Otherwise they don't change.

You can't cube it since it doesn't have potential.

You can't use normal cubes on items with only bonus potential.

If you use a bonus potential cube, well the results are obvious.