What do you do if you don't have a writing sample?

If you don't have a writing sample, write one. Although it is more work, it's important to develop a writing portfolio. Even if your written work doesn't land you the specific job, you'll at least have a writing sample ready for the next job you apply for.

If the job states that you may only submit published writing samples, you can still attempt to submit a freshly written one. However, that does qualify as failure to follow instructions and is less likely to result in a job offer.