On one hand, if you haven’t prepared and don’t feel ready, it doesn’t seem to make sense to take them.
On the other hand - you have already paid for it. Even if you already selected school to receive the scores, I think you can change/delete them. Unless you have a school that strictly requires every score from every test you’ve taken, no one needs to know you’ve taken them. And you might surprise yourself.
Does CB allow you to change a date so you can take it later, at no to minimal cost?
If the tests are optional, I would think that sending none is better than sending poor scores.